The England international got on the end of Kiko Seike’s pass and her shot hit the post before nestling in the back of the net.
Katrina Gorry had scored twice in five minutes to draw West Ham level, with the Hammers captain finding the net from long range on both occasions.
Rachel McLauchlan scored her first goal for Brighton in the first half, latching on to a delightful flick from Kirby before cutting inside and curling her shot into the bottom corner.
McLauchlan then provided an assist for Seike whose shot found the net from close range.
Gorry’s quick-fire double looked to have earned the Hammers a point before Kirby’s decisive intervention.
The result means Brighton remain third in the table while West Ham are ninth.
Brighton return to winning ways
Brighton’s victory was much-needed as they bounced back from a disappointing defeat against Arsenal last time out.
McLauchlan was influential with a goal and an assist from defence, highlighting the talent Brighton have in all areas as the Seagulls ended a three-game winless run at the Amex in front of a crowd of 4,638.
Sieke’s celebration showed how relieved she was to be back on the scoresheet after failing to net in the WSL since her debut hat-trick against Everton in September.
And the experience of Kirby, who has won the WSL eight times, proved pivotal in a game in which Brighton would have been frustrated to have dropped points after dominating for so long.
After scoring her first goals for West Ham, Australia midfielder Gorry saw her hopes of a hat-trick disappear when she was taken off on 72 minutes after injuring her left shoulder while attempting a sliding tackle on Sieke.
West Ham’s fighting spirit wilted after Gorry’s substitution and they fell to their fifth defeat of the season.
Rehanne Skinner’s side have won just one of their last 15 away games in the WSL (drawing three and losing L11) and they conceded goals in all of those matches.
If the Hammers are to avoid relegation, this is a worrying trend they must address.
