A stunning first half strike from Aaron Davies earned three points for the visitors at Dagenham and Redbridge's Victoria Road ground and condemned Hammers to their first home defeat of the season.
With several players away on international duty, coach Roger Cross was forced to field an extremely young side that included just one player with first team experience - Australian forward Richard Garcia.
Despite a lively end to end opening, neither goalkeeper was overly tested until midway through the half, when Hammers left-back Brent Tattum's flying burst down the flank climaxed with a pinpoint centre that saw Saints keeper Gareth Williams claim Youssef Sofiane's eight-yard header with a stretching save.
Seconds later, Williams was again scrambling across his goal as Garcia nodded Daryl McMahon's cross inches wide.
At the other end, Aaron Davies' downward header was collected off a bumpy Victoria Road pitch by relieved Irish goalkeeper David Forde.
Then on 34 minutes, Davies was making his presence felt again with a fierce 10-yarder that thumped back off the post.
But as Hammers counted their blessings, moments later Saints took the lead as that man Davies made it third time lucky with a spectacular, dipping 20-yard effort which scorched under Forde's crossbar.
Sofiane's awkward shot from the edge of the area was then saved at the base of his right-hand post by Williams, who then clawed away Marce Collington's angled shot to ensure that his side kept their noses in front at the break.
Seconds after the restart, Mitchell Cole rapped the bar with a vicious shot but despite having the lion's share of possession, the young Hammers simply couldn't find the final ball during a desperately disjointed second half.
With 20 minutes left, a double substitution saw Darren Blewitt and Greg Pearson replace Tattum and Collington. Within moments of his arrival, Blewitt was cautioned for a crunching challenge on Michael Green, while the towering Clive Delaney followed him into Mr Russell's book for mistimed lunge on Jay Lucas.
Two minutes from time, McMahon almost got the equaliser his industrious and determined display deserved, when he let fly with a screaming 20-yarder, but still Williams refused to be beaten as he pulled off a full length save to ensure his team's lead stayed in tact until the final whistle.
Hammers: Forde, McClenahan, Tattum (Blewitt 70), Lumsden, Delaney, Ward, Collington (Pearson 70), McMahon, Sofiane, Garcia, M Cole. Subs unused: Jackson, Bunce, Parrington, Wright.