Williamson went from villain to hero after his own goal sparked a late Swansea comeback following Wycombe's two-goal lead. Jermaine Easter and Stefan Oakes had given the visitors the lead before Williamson's deflection into his own net and a Darren Pratley strike took the game into extra-time.
But then Williamson was on hand to prod home the ball into the empty net when Chris Palmer's free-kick caused a goalmouth scramble.
It was the visitors who started the sharpest as Kevin Betsy twice saw close-range efforts saved by Swansea goalkeeper Willy Gueret.
The Swansea back-line was not helped when Trinidad & Tobago international Dennis Lawrence limped off with just 16 minutes on the clock and Easter was next to try his luck on 22 minutes with Gueret again coming out on top.
Swansea did register their first real chance on goal seven minutes later as Izzy Iriekpen came close to stabbing the ball home when Rory Fallon flicked on Kevin Austin's searching free-kick.
But the series of miss-placed passes and hasty clearances that has plagued Swansea's season were still on display and Easter was given one more chance to try his luck before the interval but the effort was saved.
However Easter was not to be denied five minutes after the interval when he did break the deadlock. It was another mistake at the back that presented the chance as Swansea struggled to deal with a simple long-ball, this time Gueret was not able to stop Easter's 20-yard effort.
Then as Wycombe continued to press, Kevin Amankwaah's foul on Easter allowed Chris Palmer to take a quick free-kick and Oakes drilled home from the edge of the box.
Swansea did respond and managed to pull one back on 70 minutes when O'Leary's effort deflected off Williamson into his own net before Pratley saved the day with his own rocket nine minutes from normal time.
But Wycombe took victory with 113 minutes on the clock when another Amankwaah foul allowed Palmer to cross and Williamson eventually took advantage to poke home from close range.