The Lions winger converted on 33 minutes to earn Neil Harris his 100th victory as Millwall manager and ensure their first opening-day win in the second tier since 2014.
Preston huffed and puffed but failed to register a single attempt on target, with Alan Browne's first-half strike that was cleared off the line and Sean Maguire's curling effort just past the post the closest they came to grabbing an equaliser.
For a side that finished just one place above the relegation zone last term, the Lions sent an early statement to suggest this campaign could see improved fortunes.
Millwall, starting with four of their eight summer signings, made a lightning start to the game – with Wallace forcing Declan Rudd into a sharp early save from 25 yards after Preston only half-cleared a Shaun Williams corner in the second minute.
Preston responded with a couple of corners of their own and the second of those almost led to the opening goal. The Lions were unable to clear their lines and Browne tried his luck from the edge of the box, but Matt Smith was in the right place to head his rasping effort behind for a corner.
The visitors were dominating the ball but it was Harris' side who continued to look the more dangerous going forward. However, Mahlon Romeo needed his blistering pace to make a last-ditch challenge on Maguire after the Lions defender was initially caught hesitating on the ball.
Just past the half-hour mark, Millwall went in front. One of those four new recruits, Connor Mahoney, worked a yard of space before picking out Wallace, who expertly volleyed past Rudd, although the former Norwich goalkeeper will feel he should have kept it out.
But Millwall's joy was short-lived as goalkeeper Frank Fielding, another of their new players, was forced off through injury – forcing Harris to introduce a fifth debutant in Bartosz Bialkowski, who joined earlier this week on loan from Ipswich.
Bialkowski was almost beaten seconds into his Lions debut as he could only watch Maguire's 45th-minute curler fly just past the far post.
The second period was relatively uneventful compared to the first and despite some late pressure from the Lilywhites, which included a penalty appeal from Maguire that the referee correctly turned down, they were unable to trouble Bialkowski as their losing run on the road stretched to four games.