A Dagný Brynjarsdóttir header and long-range Yui Hasegawa chip claimed the points at the Academy Stadium, as the hosts' disappointing run of form continued.
Though Gareth Taylor's side carved plenty of chances to level - Laura Coombs striking the post in injury time - the Hammers defended valiantly to preserve their clean sheet and killed the contest in the final seconds to inflict the home side's fourth defeat in five games.
Buoyed by the midweek Women’s FA Cup quarter-final victory over Leicester, City started brightly – Lauren Hemp providing the hosts’ main attacking outlet on the left flank.
The PFA Young Player of the Year almost broke the deadlock inside ten minutes, latching onto Janine Beckie’s far-post delivery but somehow skewing her effort just off-target after good work by Jess Park.
Filippa Angeldahl – fresh from her first City goal on Wednesday night – then unlocked the Hammers defence with a lovely clipped ball through to Caroline Weir, whose touch was exquisite but just too heavy, with Mackenzie Arnold racing out to gather.
The visitors grew in confidence as the half progressed – Claudia Walker finding herself in a shooting position on the six-yard box but unable to dig the ball from under her feet.
Hemp then spurned a glorious chance to open the scoring, cutting out Gilly Flaherty’s attempted pass across goal, racing goalward but firing straight at an outstretched Arnold when one-on-one with the shot-stopper.
City were punished five minutes before the break as the away side hit the hammerblow – Hasegawa floating a cross to the backpost for Brynjarsdóttir to nod home.
The home side responded well – Park forcing Arnold into two smart stops at the end of the half – but Taylor’s side trailed at the break.
Staring in the face of a third successive league defeat for the first time since joining the WSL, City rallied and almost found a leveller inside the opening minute of the restart when Beckie's pass found Weir, whose curling effort was parried onto Hemp but ricocheted agonisingly wide.
As the home side pressed though, the visitors threatened on the counter attack, as Hasegawa and Tameka Yallop called Karima Taieb into action twice in quick succession.
White headed wide and saw a goal ruled out for offside, before Weir fired a powerful effort just over the bar, while Yallop squandered a chance to double West Ham's lead at the other end, heading wide.
Hemp, Khadija Shaw and Coombs all both stung the palms of Mackenzie, while Coombs came closest to forcing a late equaliser, smashing off the post from the edge of the area.
West Ham made sure of the points though in the dying seconds, as Hasegawa capitalised on Demi Stokes' short backpass and lofted the ball over Taieb.
What it means
City drop to ninth in the FA Women's Super League table with one win from our opening four games. West Ham lift themselves to fourth.
How we lined up
Taylor made three changes to the side that progressed to the Women's FA Cup semi-finals with a 6-0 win over Leicester in midweek.
Stokes, Keira Walsh and Ellen White were restored to the starting XI, in place of Georgia Stanway, Vicky Losada (who missed out with an ankle injury) and Khadija Shaw - Walsh making her first start of the season.
Hemp and Park occupied the wings with captain White leading the line upfront.
CITY XI | Taieb, Beckie, Scott, Greenwood, Stokes, Walsh (Coombs 46'), Angeldahl (Stanway 67'), Weir, Park, Hemp, White (C) (Shaw 67').
SUBS | Keating, Kennedy, Mace.
WEST HAM XI | Arnold, Longhurst, Cissoko, Flaherty (C), Wyne, Stringer (Filis 90'), Brynjarsdottir, Yallop, Svitkova (Parker 81'), Hasegawa, Walker (Leon 60').
SUBS | Leat, Joel, Cairns.
- City suffered back-to-back defeats on home soil in the Women’s Super League for the first time since October 2014, having never previously lost three in a row in the top-flight.
- The result ended our 100% WSL record against the Hammers.
The first Manchester Derby of the season awaits on Saturday 9 October in a 13:30 (UK time) kick-off at Leigh Sports Village. The game will be shown live on BBC One.
Our next home games sees our Continental Cup campaign begin with the visit of Everton in the group stage on Wednesday 13 September (19:00).
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