Faiz Selemani restored parity after Mitrovic's 15th-minute opener at Prince Faisal bin Fahd Stadium, only for Jorge Jesus' hosts to roar back and be crowned champions with three games remaining.
Ahmed Al Juwaid's own goal handed the initiative back to the dominant Al Hilal, who led 4-1 at the break after Mitrovic added another to his tally and Sergej Milinkovic-Savic was on target.
The second of Mitrovic's strikes took him to a team-leading 26 league goals this term as Al Hilal breezed to another title, moving an unassailable 12 points clear of Cristiano Ronaldo's Al Nassr.
Jesus' side are unbeaten through all 31 games this season after securing their record-extending 19th top-flight crown in history, while Al Hazm need to win their last three matches to have any chance of staying up.
Al Hazm's relegation rivals Al Tai boosted their chances of survival in Saturday's earlier clash after a 1-1 draw away against Al Raed.
Al Tai are just a point away from 15th-placed Abha Club in their bid for top-flight safety after Ibrahim Al Nakhli's strike was cancelled out by Al Raed's Mohammed Fouzair in the second half.
Jorge Jesus' side finished the league campaign unbeaten across the 34-game season, though needed late heroics from top scorer Mitrovic to snatch all three points.
Mohamed Kanno opened the scoring for the visiting champions in the first half but Yahya Naji's second-half leveller pegged Al-Hilal back at King Abdulaziz Sports City Stadium.
Yet Mitrovic provided the decisive moment in the 92nd minute, converting a Mohammed Al Burayk assist for his 28th league goal of the season.
The Serbia international ranks only behind Cristiano Ronaldo in this campaign's scoring charts, after the Al-Nassr forward claimed a piece of Saudi League history in a 4-2 victory over Al-Ittihad.
Ronaldo moved past Abderrazak Hamdallah's 34-goal tally – in the 2018-19 season – after his deadly double took him to 35 strikes and broke the single-season scoring record in the competition.
Victory for Al-Nassr means they finish second behind the impressive Al-Hilal, while Al-Ahli had already secured third in the standings before their 1-0 victory over Al-Fayha.
Roberto Firmino scored the only goal of the game in that narrow triumph, finding the net after 85 minutes at Prince Abdullah al-Faisal Stadium.
At the other end of the table, Abha Club – who had Marcel Tisserand sent off late on – suffered heartbreak on the final day after losing 2-1 to bottom-placed Al-Hazem to seal their relegation.
Ahmed Abdu opened the scoring to boost Abha's survival hopes but second-half goals from Muhammed Badamosi and Mohammed Al Thani proved their undoing.
Al-Akhoud beat the drop after overcoming Al-Tai, who went down with Abha and Al-Hazem, 2-0 thanks to goals from Saviour Godwin and Leandre Tawamba at Prince Abdul Aziz bin Musa'ed Stadium.
