Elisabeth Terland scored in the ninth minute. Andrea Medina added a second in the 72nd. Viviane Asseyi converted a penalty in the second minute of stoppage time.
United had one point from three league matches after a 1-2 defeat at London City Lionesses, a 0-5 home loss to Chelsea and a 1-1 draw at Arsenal. Eva Olid therefore has her first WSL win as manager.
Terland finished from close range after a corner. Medina doubled the lead after the break. West Ham stayed in the contest and were awarded a late penalty, which Asseyi scored. United held on.
The result lifts United to four points from four games. West Ham have three points from four. United host Liverpool next in the league.
A midweek 10-1 Players Cup win over Sheffield United had shown attacking depth. Sunday’s three points were the ones that mattered after a difficult start in the Super League.