After Sergio Aguero had an overhead kick blocked on four minutes, Sterling opened the scoring just before the half-hour.
The winger finished off Leroy Sane's cross after a quick counter-attack.
Aguero got his fifth PL goal of the season net with 25 minutes remaining, after a brilliant one-two with Sterling.
Brighton slip to 14th with five points.
Man City: 7 Oct v Liverpool (A)
Brighton: 5 Oct v West Ham (H)