Top scorer Stuart Beavon put the hosts ahead in the first half but Williams helped to turn the match on its head as the Glovers dropped their opponents back into the bottom four.
Wycombe went ahead against the run of play in the 15th minute when a slip by Richard Hinds let in Matty Wichelow, whose lay-off allowed Beavon to finish into the bottom corner.
That was how it remained until Yeovil equalised just before the hour mark when Jonathan Obika's low cross was touched by Chairboys goalkeeper Nikki Bull only for Williams to tap into the empty net.
Eight minutes later and the Glovers were ahead as Williams burst into the area and sending in a low ball which evaded Obika and trickled into the far corner.
Substitute Kieran Agard then doubled the visitors' lead by nodding in Ed Upson's free-kick in the 73rd minute, before Joel Grant gave Wycombe brief hope by sliding the ball home after Matt Bloomfield's throughball.
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