Teenage stars Alejandro Garnacho and Kobbie Mainoo struck in the first half at Wembley and United then dug deep to win 2-1 after City substitute Jeremy Doku's late effort. It means Ten Hag's side finished a miserable season on a high and also qualified for next season's Europa League.
The build-up to the second successive all-Manchester FA Cup final was dominated by talk over Ten Hag's future after a report on Friday said the Dutchman would be sacked regardless of the outcome at Wembley. United finished eighth in the Premier League - their lowest placing since 1990 - and crashed out of the Champions League at the group stage.
Ten Hag admitted that he still did not know what the future would hold for him. "I don't know (if I will stay)," he told the BBC as he celebrated United's win in the London sunshine.