Fletcher hit a top score of 69 to help the Falcons produce 187-9 from their 20 overs after being put in to bat by the Marvels.
His knock lasted 43 balls and included six fours and four sixes. He got good support from opening partner Dinesh Chandimal who made 32 while Angelo Mathews and Ramesh Mendis chipped in with 29 and 28, respectively.
Prabath Jayasuriya took 3-30 from his four overs for Galle.
The Marvels then took 19.2 overs to reach 188-2 and secure a comfortable victory.
Alex Hales was the catalyst behind the successful chase with 86* off 55 balls including seven fours and four sixes while Bhanuka Rajapaksa finished 46* off 25 balls including five fours and a pair of sixes.
Captain and wicket-keeper Niroshan Dickwella also chipped in with 25.
Fletcher also took 1-11 from two overs with his part-time leg-spin.
The Marvels are now second in the points table after six games, level on eight points with leaders Jaffna Kings.
The Falcons have two wins from six games and sit last on the six-team table.
Full Scores:
Kandy Falcons 187-9 off 20 overs (Andre Fletcher 69, Dinesh Chandimal 32, Angelo Mathews 29, Ramesh Mendis 28, Prabath Jayasuriya 3-30, Dwayne Pretorius 2-37)
Galle Marvels 188-2 off 19.2 overs (Alex Hales 86*, Bhanuka Rajapaksa 46*, Niroshan Dickwella 25)
Lasith Embuldeniya 5-35 and Man-of-the-Series Ramesh Mendis 5-66 bamboozled the West Indies, which had gone to lunch at 65-2 after facing 30.4 overs in the pre-lunch session. However, on the resumption, the Caribbean men lost their remaining eight wickets for 67 runs.
Nkrumah Bonner made 44 in another defiant performance.
After Kraigg Brathwaite was dismissed for six with the score on 15, Bonner and Jermaine Blackwood had mounted a partnership of 50 runs without much drama until Blackwood was out on the stroke of lunch for 36.
After lunch, Shai Hope (16) and Kemar Roach (13) were the only batsmen to make double figures as the Sri Lankan spinners cut through the lineup that offered little resistance as the West Indies were all out in 25.3 overs after the interval.
Resuming after lunch at 65-2, Hope and Bonner had taken the score to 92 when Mendis took three wickets - Hope, Roston Chase (0) and Kyle Mayers (0) - in the 44th over.
Holder (3) and Bonner followed shortly thereafter as the West Indies slipped further from 92-5 to 108-7 before Roach and Joshua da Silva, who remained unbeaten on four, attempted to mount another lower-order stand. However, when Roach became Embuldeniya's fourth victim at 128, it did not take long for Sri Lanka to wrap up the innings as Mendis claimed Veerasammy Permaul for one and Embuldeniya delivered the final rites when he dismissed Jomel Warrican for three.
Earlier, Sri Lanka had resumed from their overnight score of 328-8 with Man of the Match Dhananjaya de Silva on 153 and Embuldeniya on 25. The pair extended the ninth-wicket partnership of 107 to 126 before Embuldeniya was bowled by Jason Holder for 39.
De Silva remained not out on 155 as Sri Lanka declared at 345-9.
Holder finished with figures of 1-26. Veerasammy Permaul, who had eight wickets in the match, took 3-106 and Roston Chase 2-82.
Sri Lanka won the first Test by 187 runs.
Led by Captain Kraigg Brathwaite's patient 72 and propped up Kyle Mayers' crucial unbeaten 36, the West Indies scored 253 all out in reply to the home side’s first-innings score of 204.
The visitors had resumed from their overnight score of 69-1 with Brathwaite on 22 and Nkrumah Bonner on one. The pair extended their overnight partnership of seven runs to 75 when Ramesh Mendis claimed the first of his six wickets trapping Bonner lbw for 35.
The visitors went to lunch at 145-2, still 59 runs behind the hosts but they failed to build a commanding lead having lost wickets at regular intervals on the resumption after Brathwaite was bowled by Lasith Embuldeniya.
Shai Hope’s miserable Test form continued as he made only 10 while Roston Chase made 22 as the West Indies slipped from 166-3 to 197-7 as Mendis inflicted further damage on the West Indies batting order.
However, Mayers' late innings heroics proved instrumental in giving the West Indies a 49-run lead with his unbeaten knock that guided the West Indies to a small but important advantage.
Mendis’s six wickets cost 70 runs while Embuldeniya took 2-94 and Praveen Jayawickrama 2-59.
Trailing on first innings for the first time in the series, Sri Lanka got off to a bad start losing their leading run-scorer for the series, Dimuth Karunaratne, who was run out for six with only seven runs on the board. Oshado Fernando was also run out, for 14, as the home team crawled to 39-2 as close of play approached.
However, at stumps, Pathum Nissanka remained unbeaten on 21 with debutant Charith Asalanka at the other end on 4.
The West Indies trail 0-1 in the two-Test series.
Ramesh Mendis, who took 3-23 and Praveen Jayawickrama 2-25 were the main destroyers for the hosts as the West Indies collapsed from a solid start from Captain Kraigg Brathwaite, who made 41 and Jermaine Blackwood (20).
Brathwaite and Blackwood were hardly troubled by the Sri Lankan bowling as both took few risks while making effort to rotate the strike as best they could against tight-bowling and a close field. They ran singles and pierced the field for the occasional boundary steadily building a foundation for the chase.
However, the promising start would eventually unravel in the most disappointing fashion.
At 46-0, Blackwood was trapped lbw playing down the wrong line to a Lasith Embuldeniya delivery that was going on to hit leg stump. Nkrumah Bonner was out soon after as after having faced 11 balls, Jayawickrama had him caught at slip for one.
His dismissal triggered a slide that saw the West Indies lose four wickets for 53 runs. Among the wickets to fall was that of Brathwaite, who was caught at leg slip off Mendis for 41 as close of play approached.
The wickets of Shai Hope (10), Roston Chase (2) and night watchman Jomel Warrican followed in quick succession as the West Indies slipped further from 80-3 to 100-6.
Kyle Mayers 22* and Jason Holder 1* are the batsmen at the crease when play ended for the day.
Earlier, Chase took 5-83 to restrict Sri Lanka, resuming from their overnight total of 267-3 were bowled out for 386.
The hosts lost their last seven wickets for 119 runs after overnight batsman Dhananjaya de Silva, who resumed from his overnight score of 56, added five runs to his score before he was first out, hit wicket to the bowling of Shannon Gabriel for 61.
The big wicket of Dimuth Karunaratne, 132 overnight, followed soon after when he was caught at slip for 147.
Chase also removed Dinesh Chandimal for 45 as he and Jomel Warrican (3-87) ran through the remainder of the Sri Lanka batting.
