The Easter Road favourite drove a shot past Vaclav Hladky five minutes from time to seal a 1-0 result.
It had been a frustrating afternoon for the hosts up until that point as they struggled to score through a combination of poor finishing, the post and the offside flag.
Saints played their part too, with winger Ilkay Durmus making Hibs goalkeeper Ofir Marciano work.
Hibs started with a total of five new summer recruits in the side, while St Mirren handed full debuts to Sam Foley and Sean McLoughlin following their arrival in midweek.
The teams prepared for the opening league match on the back of contrasting Betfred Cup campaigns which saw Hibs progress and St Mirren fail to reach the last 16.
The Leith side attempted to build on that momentum after just five minutes as Stevie Mallan's fierce left-footed shot from 20 yards rippled the top of the net.
Paul Heckingbottom's side continued to press but had to be wary of the Buddies on the counter.
Saints winger Durmus had a low drive comfortably saved by Marciano before the same player forced an acrobatic block by the Israeli with another effort inside the area.
Hibs responded with drives from distance from Mallan and Steven Whittaker that were kept out by Hladky.
The best chance of the half fell to Durmus again three minutes before the break but the German-born Turkish playmaker lashed a half-volley over the bar from 12 yards.
Hibs started the second half with greater purpose but were denied by the post and the offside flag in their bid to find the opener.
From an acute angle in the area, Allan curled a sublime shot around the goalkeeper only for his effort to hit the upright and roll back across the line.
Joe Newell then directed a header off the same post from Whittaker's cross before Flo Kamberi tapped home the rebound.
However, the Swiss' joy was cut short by the assistant referee's flag.
St Mirren boss Jim Goodwin handed striker Jonathan Obika his debut from the bench and the former Tottenham youngster set up Tony Andreu outside the area, but the midfielder's stinging drive was beaten away by Marciano.
An even better chance for Hibs came and went in the 69th minute. Kamberi had a low drive parried by Hladky from an exquisite Allan through pass before substitute Christian Doidge got the ball stuck under his feet with the goal at his mercy, allowing Saints to clear.
St Mirren defender McLoughlin then headed just wide at the back post and that miss came back to haunt the visitors as Hibs scored five minutes from time through Allan.
The midfielder drove inside the area from a Doidge pass before slamming a shot past Hladky.
Doidge should have sealed the points but failed to make proper contact with Kamberi's pass from just two yards out.