Experienced frontrunner Williams, out for more than a year with injury prior to the start of this season, headed in at the far post Lucy Staniforth’s corner 17 minutes from time having added a timely boost to Blues’ attack.
Manager Marta Tejedor made one change from the 1-0 defeat at West Ham, handing Connie Scofield a first start of the season with Chloe Arthur on the bench.
The action took time to develop with Blues fashioning the first opening when Staniforth’s free-kick flashed off the head of Kerys Harrop rather than the captain connecting properly 14 minutes in. Everton’s first moment of note came soon after when Taylor Hinds flicked into the run of Chloe Kelly on the right but Blues keeper Hannah Hampton covered the near-post dash.
Kelly pulled the trigger again a minute later with Hampton having to adjust to deal with a deflection and Gabby George’s cutback to Danielle Turner was well dealt with by home defender Adrienne Jordan.
Blues steadily turned the tide with Brianna Visalli pouncing on some loose play at the back before becoming embroiled in a tangle that saw her penalty appeal waved away by referee Lucy Oliver.
Scofield had a shot charged down after a burst down the right by Sarah Mayling, Visalli’s 20-yard swinging effort dropped wide of the post and tidy approach play between Claudia Walker, Staniforth and Scofield forced Hinds to head over her own bar as the pressure built before the break.
A series of Blues corners saw Harrop denied by a block after Kirstie Levell had spilled one of the set pieces, although Hampton had to be alert at the other end to charge out at Turner’s feet after Harriet Scott’s backpass fell short.
Blues remained on the front foot after half-time with Harrop a fraction away from connecting with Staniforth’s inswinging free-kick. Visalli’s cross-shot again dropped to Harrop six yards out but the defender could not quite sort her feet out, while Mayling had a header blocked and cleared by George.
Blues kept probing and eventually got their breakthrough when Williams burst through the crowd to make the net ripple for the first time this season from Staniforth's hanging delivery.
Buoyed by the goal, pressure from Williams led to Abbi Grant being gifted space on the left of the box but her cross-shot ended up being neither with Williams in support at the far post.
That same connection saw Blues go within a whisker of a second with Williams flicking on Hampton’s long drive forward for Grant but the high shot across goal smacked the face of the bar with Levell beaten.
There were a couple of late scares as Everton substitute Hannah Cain was slipped down the side of the box only to be met by the alert Hampton who then pushed away a cross-shot from Turner that was destined to drop under the bar in added time.
Blues: Hampton, Scofield (Arthur 64), Harrop, Holloway, Visalli, Grant, Jordan, Staniforth, Walker (Williams 57), Mayling, Scott (Whipp 74).
Unused subs: Logan, Maguire (g/k). Goal: Williams 73.
Everton: Levell, Hinds (Van Es), Turner, Brougham, George, Kaagman, Hughes (Cain 62), Kelly (Pike 71), Graham, Finnigan, Clemaron. Unused subs: Morgan, Bryson, Korpela (g/k).
Bookings: Kelly 45+1, Van Es 55.
Referee: Lucy Oliver.