Andriy Yarmolenko’s goal shortly before half-time and a wonderful late Aaron Cresswell free-kick extended United’s barren run on the road.
United, who started the season by thumping Chelsea 4-0, have lost twice already in their opening six league games and lie seventh, without an away win in any competition since March.
Injury-hit United were without midfielder Paul Pogba and forward Anthony Martial and huffed and puffed but failed to break down their dogged opponents.
West Ham are riding high under Manuel Pellegrini, unbeaten since a 5-0 drubbing in the first match of the season and this victory followed a home win last season against United at the London Stadium.
But it was a chastening day for Ole Gunnar Solskjaer’s men, just a day after champions Manchester City came close to matching United’s Premier League record for margin of victory — 9-0 against Ipswich in 1995. (AFP)