The 26-year-old found the top corner with an unstoppable set-piece to level the scores four minutes after Aden Flint had put the hosts in front from close range.
Wallace's seventh goal from his last 13 matches ensured his side remained level on 34 points with Cardiff, who appointed former Lions boss Harris in November.
Harris' successor Gary Rowett made only a single change to the Millwall side who saw their seven-game unbeaten streak ended by Barnsley last weekend as centre-back Alex Pearce replaced winger Jiri Skalak.
The alteration signalled a change to a 3-4-3, the same system that proved so effective when they snatched victory at Swansea in their last trip over the Severn Bridge.
The last four matches between these sides had ended in draws and this one looked destined to finish the same way after a drab first half.
The only drama came in the closing stages of the opening 45 minutes when Millwall were denied a penalty.
Wallace broke downfield three minutes before half-time and laid the ball off for Tom Bradshaw to have a shot at goal.
Cardiff defender Curtis Nelson scrambled back and took Bradshaw down without getting a touch on the ball, but referee David Webb waved away the protests for a penalty.
Cardiff striker Robert Glatzel shimmied past Jake Cooper and unleashed a fierce shot just before the half-time whistle, but Bartosz Bialkowski stood strong to ensure his side went in level at half-time.
The Lions goalkeeper went one better the other side of the interval with a stunning save to deny Lee Tomlin.
The Poland international went full stretch and used his fingertips to divert Tomlin's left-footed volley just past the post.
He was powerless to stop the Bluebirds from taking the lead after 60 minutes though.
Left-back Bennett swung in a corner that was flicked on by Marlon Pack at the front post, and there was nobody tracking the run of Flint as he ghosted into the six-yard box unmarked to tap the ball in.
But Wallace hauled Millwall back level within four minutes with his ninth goal of the season.
Pack pulled down Shaun Williams to give Wallace a dangerous dead-ball opportunity and he fired an unstoppable free-kick past Neil Etheridge into the top corner to level the scores.
The game opened up as both sides went in search of a winner, but they were forced to settle for a share of the points.