The 20-year-old was forced to settle for a place on the bench after rejoining the Tractor Boys on loan from Crystal Palace on Monday but he made up for lost time with a nice finish just minutes after his introduction.
Atdhe Nuhiu had earlier given Wednesday a dream start with his fifth goal of the season, only for Williams to extend their winless run against Ipswich at Hillsborough to six games.
The Owls went into the contest fresh from back-to-back defeats in the league and cup but they were quickest out of the blocks against their in-form visitors.
Dean Gerken did well to push Chris Maguire’s low shot behind but was powerless to keep out Nuhiu’s near-post header from the resultant corner delivered by Jacques Maghoma on five minutes.
Maguire dragged a 25-yard strike wide and David McGoldrick scuffed an effort straight at Keiren Westwood as Ipswich strived to get back into the game.
Wednesday thought they had doubled their lead when Kamil Zayatte nodded home after a brilliant reflex save by Gerken, only for referee Darren Bond to blow for an infringement.
Zayatte suffered another slice of misfortune nine minutes from the break, appearing to pull a hamstring as he put the ball out for a throw-in.
Ipswich went agonisingly close to grabbing an equaliser as half-time approached as Daryl Murphy headed Tyrone Mings’ pinpoint cross inches wide.
There was still time for Gerken to smother another Maguire shot and Murphy to send a looping header over the bar.
Wednesday had the ball in the net for a third time in the opening few minutes of the second period but the offside flag was raised long before Nuhiu converted at the second attempt.
Williams was introduced by Mick McCarthy and the change proved a masterstroke as the Wales international made it all square in the 62nd minute.
He entered the field without a number on his back but ensured he would not go unnoticed as he took the ball down on his chest at the edge of the box before hitting a low shot beyond Westwood and in off the far post.
The impressive Maguire almost put Wednesday back in front within minutes, his deflected shot fizzing narrowly wide of the post.
Stevie May, who was handed his first call-up to the full Scotland squad earlier on Tuesday, headed a Maghoma cross wide and Christophe Berra sent one just over at the other end as both sides pushed for a winner.
Williams was brought off in the closing minutes but he had done his job as the points were shared.