Cummings opened the scoring in the 22nd minute and ten minutes later took advantage of a horrific mistake by Saints defender Jack Baird to claim another.
Incredibly, it was the first time Hibs have won their opening three league games in 42 years, but it was still not enough to take them to the top of the table, Raith Rovers still out on front but only on goal difference after winning the Fife derby 2-0 against Dunfermline.
Hibs reverted to a back four with Liam Fontaine out with a calf injury as Dylan McGeouch returned to the starting line-up, while new signings Andrew Shinnie and Brian Graham took a seat on the bench where they were joined by Fraser Fyvie, now over the knee problem which had kept him out since the start of the season.
A surging run from James Keatings create the game’s first chance. Cummings’ shot was blocked with the ball falling for Gray, who returned it to the danger area. John McGinn picked it up in space, but the midfielder was unable to direct his shot on target.
Hibs, however, did take the lead midway through the first half with a well-crafted goal. McGinn won possession inside his own half for McGeouch to feed it on to Keatings who, in turn, found Gray in plenty of space on the right. The Hibs skipper sucked in Lewis Morgan before leaving him trailing and then sending in an inviting cross which Cummings turned into the net from close range.
Hibs hadn’t been threatened at the back, but McGinn was caught napping by Lewis Stevenson’s throw-in, losing possession to Kyle Hutton who laid the ball off for Stevie Mallan to send in a dipping shot which just cleared Ross Laidlaw’s crossbar.
The Edinburgh club, though, doubled their lead in the 32nd minute. Darren McGregor’s ball forward didn’t appear to carry any threat, but Baird’s attempt to pass it back to Scott Gallacher went wide of his goalkeeper, leaving Cummings to slide it home from a tight angle.
A third for Hibs would have all but killed the game off and it almost came when McGinn spotted Cummings in space, but the striker’s weak right foot shot was easily held by Gallacher, who then kept his side in it by blocking another effort from the Easter Road hitman and then recovering to prevent Grant Holt netting the rebound.
That two-goal cushion left Hibs very much in control of the match and they never looked like loosening their grip in the second half, although boss Neil Lennon’s frustration that they couldn’t stretch that advantage was clear to see on more than one occasion.
The situation, though, did allow the Hibs head coach to give both Shinnie and Graham a run-out, the on-loan Birmingham City midfielder going close with a dipping effort.
At the other end, substitute David Clarkson’s shot was well held by Laidlaw, while in the final minute only Gallacher’s crossbar prevented McGinn claiming the third goal they richly deserved.
St Mirren: S Gallacher, Naismith, Baird, Webster, Irvine, Walsh, Hutton, Mallan, Morgan (Hardie 46), Sutton (Clarkson 70), Shankland (Stewart 75). Substitutes not used: Langfield, Whyte, Gordon, McDonald.
Hibs: Laidlaw, Gray, McGregor, Hanlon, Stevenson, Bartley, McGeouch, McGinn, Keatings (Shinnie 69), Cummings (Boyle 84), Holt (Graham 76). Substitutes not used: Virtanen. Fyvie, Forster, Crane.
Referee: Don Robertson.