The MLS expansion side had suffered back-to-back losses after creating league history by winning their first five games but responded at CityPark.
Jared Stroud fired in a third-minute opener from a set-piece before Eduard Lowen lashed home a volley in the 39th minute.
Kyle Hiebert glanced in Stroud's cross in first-half stoppage-time and St Louis did not relent after the break, with Nicholas Gioacchini adding another from Rasmus Alm's classy touch in the 53rd minute.
It was five in the 57th minute when Alm's rifled strike from outside the box hit the post and deflected in off goalkeeper Roman Celentano.
Sergio Santos grabbed a consolation for Cincy, who were unbeaten coming into this encounter.
Dairon Asprilla netted an early goal of the year contender with an acrobatic bicycle kick to equalise for Portland Timbers before they raced away with a 4-1 win over Seattle Sounders.
Asprilla brilliantly netted in the 71st minute before Portland added a further three goals in the final 20 minutes, meaning Seattle slipped from top spot in the West.
Sean Zawadzki netted a 90th-minute equaliser as Columbus Crew pinched a 1-1 home draw with 10-man New England, who move up to top spot in the Eastern Conference ahead of Cincinnati.
Lorenzo Insigne returned from a groin injury as Toronto drew 2-2 with Atlanta. Toronto have drawn four games in a row.
DC United ended their six-game winless run with a 1-0 victory over last-placed Montreal, while New York Red Bulls' streak without a win extended to four games after a 1-1 draw with Houston Dynamo.
Keaton Parks and Maxime Chanot scored excellent early goals as New York City moved up to fifth in the East with a 2-1 win over Nashville.
Austin drew 0-0 with Vancouver Whitecaps, while Orlando City rallied from behind to get past Minnesota United 2-1.
Bernard Kamungo scored an 88th-minute winner as Dallas moved up to fourth in the West with a 2-1 win over Real Salt Lake, while San Jose Earthquakes crushed Sporting KC 3-0 with a Jeremy Ebobisse double.
Struggling Charlotte drew 2-2 with Colorado Rapids and Philadelphia Union's winless run extended to five games with a 2-2 draw at Chicago Fire although they fought back from 2-0 down.
City had made MLS history by winning their first five games but that was ended by the Loons with Amarilla drilling home the spot kick after a needless challenge by Kyle Hiebert.
Hiebert brought down Amarilla just inside the box with a rash tackle when Jakob Nerwinski appeared to have Kervin Arriaga's wayward pass under control.
Substitute Samuel Adeniran came closest to equalising for St Louis, with a 95th-minute shot going over the bar after a great first touch from Klauss' right-wing cut back.
St Louis remain top of the table with 15 points from six games, but unbeaten Minnesota climb up to fourth in the Western Conference.
Cincinnati continued their impressive start to the season with a 1-0 home triumph over Inter Miami who slumped to their fourth straight defeat.
Yerson Mosquera scored the 45th-minute winner for Cincy, who are top of the Eastern Conference.
Giorgos Giakoumakis scored an early winner as Atlanta United moved up to second in the East with a 1-0 home win over New York Red Bulls, who are down in 12th in that conference.
Jordan Morris and Leo Chu scored first-half goals as Seattle Sounders stayed second in the West with a 2-1 victory at LA Galaxy, who are without a win and have only three points from five games.
LAFC remained unbeaten and third in the West despite a 0-0 draw at the winless Colorado Rapids, while SJ Earthquakes climbed to fifth with a 2-1 victory over Houston Dynamo with all three goals coming from penalties.
Vancouver Whitecaps snapped a run of three straight draws with a resounding 5-0 win over 10-man Montreal with Simon Becher scoring a double. Becher is the fastest player (87 minutes) in MLS history to score his first four goals.
Franck Boli rescued a 1-1 draw for Portland Timbers at Dallas, Chicago Fire and DC United drew 0-0 and Nashville triumphed 2-0 at Orlando City.
Federico Bernardeschi netted another spectacular goal with a strike straight from a corner in Toronto's 2-2 home draw with Charlotte.
Sporting KC remained winless after a 0-0 draw at Philadelphia Union, New England drew 1-1 with New York City and Real Salt Lake suffered their fourth straight loss, going down 4-0 to Columbus Crew.
Goals from Lewis Morgan and Luquinhas secured the three points for NYRB, with Morgan scoring his seventh for the season, heading in from a corner in only the second minute.
Luquinhas assured the three points in the 56th, spectacularly chipping over Quentin Westberg from outside the penalty area after Tom Barlow was initially denied when put through one-on-one.
With only one win in their previous six games, the three points were critical for Gerhard Struber's side.
The Philadelphia Union could only secure a 1-1 draw at home to Cincinnati, meaning they join New York City on 26 points at the top of the Eastern Conference, but City are still to play on Sunday against the Colorado Rapids.
An Ercan Kara brace helped Orlando City move into fourth position in the East, backing up from Wednesday's draw in New England to beat Houston Dynamo 2-1.
They leapfrogged CF Montreal, who lost 1-0 to a ten-man Austin FC, following Maxi Urruti's goal in the 67th minute.
Elsewhere in the East, Chicago Fire defeated DC United 1-0 in the battle between the conference's bottom-placed teams, while Columbus Crew and Charlotte played out a 1-1 draw.
There were also stalemates at the top of the Western Conference, with Los Angeles FC and the Seattle Sounders managing a 1-1 draw, as well as Los Angeles Galaxy and Portland Timbers.
It allowed Real Salt Lake to move two points from LAFC, following their 2-0 victory over the San Jose Earthquakes in Saturday's final game.
Salt Lake captain Marcelo Silva opened the scoring in the 22nd minute after Jefferson Savarino's flick-on from a corner, and Savarino doubled the margin in the 81st with a fine solo effort.
The Vancouver Whitecaps stayed within touching of the West's playoff spots, meanwhile, defeating FC Dallas 2-0.
Union took an early lead after Lukas MacNaughton's 16th-minute own goal before Denmark international Uhre bagged a double before half-time.
Uhre added a third in the 56th minute to round out his hat-trick, tapping in Julian Carranza's low ball across goal and put the contest to bed.
Toronto pulled one back through Italy international Lorenzo Insigne in the 66th minute, before Richie Laryea's stoppage-time consolation.
New England reinforced their grip on top spot in the Eastern Conference with a 2-1 home win over Sporting KC, who are last overall.
Giacomo Vrioni netted a first-half brace for the Revs, but Sporting pulled one back in the 50th minute from Johnny Russell, only to be reduced to 10 men when Andreu Fontas was sent off 10 minutes later.
The Revs are ahead of second-placed Cincinnati on goal difference, with Cincy winning 2-1 over Portland Timbers after strikes from Sergio Santos and Brandon Vasquez.
New York City moved up to fourth in the East with a 3-1 home win over Dallas sparked by Santiago Rodriguez's double.
Western Conference leaders St Louis City were denied victory after Michael Barrios' 90th-minute equaliser as they drew 1-1 at Colorado Rapids.
St Louis remain top in the West but only on goal difference from Seattle Sounders who condemned Minnesota United to three straight losses with a 1-0 victory.
Denis Bouanga netted his seventh goal of the MLS season as LAFC drew 1-1 at Nashville, while LA Galaxy snapped their winless start to the campaign with a 2-0 home victory over Austin.
Inter Miami's losing run extended to six in a row, compounding the pressure on boss Phil Neville, as they went down 1-0 at Houston Dynamo.
Real Salt Lake beat San Jose Earthquakes 3-1, Charlotte defeated Columbus Crew 1-0, struggling Montreal edged New York Red Bulls 2-0 and Christian Benteke scored as DC United won 3-1 at Orlando City.
A Maximiliano Urruti double within the opening half-hour put Austin in a commanding position at the interval, but they struggled in the second half, giving up eight shots to three despite 62.3 per cent of possession.
Sebastian Driussi clinched the result for Josh Wolff's side in the 68th minute, though, getting on the end of a flick-on from Nick Lima's long-throw at the back post to head home.
Austin have now gone five games undefeated in the league, bouncing back from Wednesday's elimination at the hands of San Antonio FC in the US Open Cup.
They remain a point ahead of the Los Angeles Galaxy, who scraped a 1-0 win at home to Nashville via Dejan Joveljic's winner in the 86th minute. Real Salt Lake slipped up away to Portland Timbers, playing out a goalless draw.
There were also goalless draws between Sporting KC and Columbus Crew, and Colorado Rapids and Charlotte FC.
They were leapfrogged by FC Dallas, who claimed their fourth win of the season with a 2-1 win over Houston Dynamo.
They left it very late in the Texan derby after Sabastian Ferreira's opener for Dynamo in the 33rd minute, only equalising in the 87th minute through Tsiki Ntsabeleng.
Facundo Quignon claimed the three points for the Toros, lurking unmarked at the back post from a corner to score in the third minute of injury time.
Philadelphia Union remained top of the Eastern Conference despite a 1-1 draw at home to Montreal CF, giving the impetus to Orlando City and New York RB to make up ground on Sunday.
San Jose Earthquakes moved from the bottom of the Western Conference with a shock 4-3 win over the Seattle Sounders. Cristian Espinoza scored the winner for his hat-trick in the fourth minute of injury time via the penalty spot.
In Saturday's other results, Minnesota FC defeated Chicago Fire 3-0, while DC United claimed a 3-2 win over New England Revolution.
The Earthquakes trailed the Whitecaps 3-1 after 59 minutes in Orlando, Florida and were staring at their first Group B defeat on Wednesday.
But the Earthquakes – who drew their opening match – rallied midweek thanks to veteran striker Chris Wondolowski, Oswaldo Alanis and substitute Salinas' memorable 98th-minute solo run.
Opening their MLS is Back Tournament campaign, the Whitecaps made a fine start as they raced out to a 2-0 lead at Walt Disney World resort.
Ali Adnan's curled effort from the top of the penalty area put the Whitecaps ahead in the seventh minute before a Judson own goal following a swift counter-attack doubled Vancouver's lead 15 minutes later.
The Earthquakes gave themselves some hope on the stroke of half-time when Andres Rios reduced the deficit, only for Cristian Dajome to restore Vancouver's two-goal buffer approaching the hour-mark following a goalkeeping error.
Wondolowski came off the bench and kept San Jose in the contest with 18 minutes remaining – the league's all-time leading scorer improving his tally to 160.
The Earthquakes then drew level through Alanis nine minutes later before Salinas' moment of magic completed the breathtaking comeback.
It was all Galaxy in the first half, controlling 71 per cent of the possession while not allowing a single shot on target, but that all changed after what must have been an inspired half-time team talk in the Vancouver dressing room.
While Los Angeles still had the bulk of the possession in the second half (64 per cent), it was the Whitecaps generating the chances, with six shots on target to the Galaxy's two.
It took a corner to produce the opening goal, with an initial sharp header from close range being saved by Galaxy goalkeeper Jonathan Bond, before Ryan Gauld was able to force home the rebound in the 57th minute.
The advantage was doubled 11 minutes later when an unsuccessful Los Angeles corner launched a Vancouver counterattack, with a cross into the near post being expertly flicked on for Pedro Vite to tap-in at the back post, marking his first MLS goal.
Tosaint Ricketts came off the bench to complete the rout, getting behind the defence and finishing off a one-on-one in the 88th minute to trim the gap between Vancouver and the last Western Conference playoff spot to six points.
Meanwhile, it was the first time since July that Atlanta United have kept a clean sheet as they left Orlando City with a 1-0 result.
The only goal of the game – despite a combined 30 shots – came in the 72nd minute when Thiago Almada snaked his way into the penalty box, getting on the end of a one-two with Andrew Gutman before tucking away his match-winning finish.
Atlanta are now two points out of an Eastern Conference playoff spot, while Orlando City are one point inside that line, while also having a game in hand.
Austin FC strengthened their position as the West's second-best team, comfortably handling the challenge of fellow playoff side Real Salt Lake 3-0 at home.
Just like Vancouver, Austin piled on all of their goals in the last half-hour, with Moussa Djite bagging a hat-trick as he came on in the 58th minute and struck in the 60th, 76th and 80th.
In the final game of the day, the San Jose Earthquakes remained stuck to the bottom of the table as they went down 2-1 on the road against the Colorado Rapids.
Vela struck twice within six first-half minutes, first from the penalty spot before getting in behind the Colorado defence and calmly finishing past goalkeeper William Yarbrough.
The former Mexico international produced his best goal of the game to complete his hat-trick in the 50th minute, curling in a fine left-footed shot from just inside the box.
Vela only managed five goals from 20 MLS appearances last season with LAFC head coach Steve Cherundolo cautiously optimistic about his potential impact this term.
“We will have to wait and see what it does. But I think we all know what he can do,” Cherundolo said. “He showed it again today and he worked very hard in preseason to get to where he is today. I'm very happy for him and the group that we're all rewarded for it.”
Yimmi Chara's spectacular 78th-minute goal earned Portland Timbers a 2-2 draw with 2021 Supporters' Shield winners New England Revolution.
Portland, who reached the MLS Cup final last year, found a way back with Chara firing home a stunning overhead goal while Jozy Altidore debuted for the Revs as a 79th-minute substitute.
Ex-Liverpool and Bayern Munich midfielder Xherdan Shaqiri made his debut for Chicago Fire as they drew a blank in a 0-0 draw at Inter Miami.
Leonardo Campana's 10th-minute header from Gonzalo Higuain's cross hit the crossbar for Miami, while Chicago's Stanislav Ivanov struck the upright in the 31st minute. Shaqiri was impressive throughout, with a long-range first-half chance his only major goal attempt.
Toronto, who made a major off-season signing with Lorenzo Insigne to join in July, only managed a 1-1 draw at Dallas.
Philadelphia Union started its new season with a 1-1 home draw against Minnesota United, with Cory Burke levelling Robin Lod's 23rd-minute strike.
New York Red Bulls won 3-1 at San Jose Earthquakes, while Columbus Crew swept aside 10-man Vancouver Whitecaps 4-0.
Cecilio Dominguez netted a double as Austin won 5-0 over last year's bottom side Cincinnati, while Michael Estrada scored a brace as DC United eased past newcomers Charlotte 3-0.
