Cummings was in the right place at the right time to convert David Gray's cross for his first.
And the 21-year-old rolled in an opportunistic second from a tight angle when he capitalised on a mix-up in the Buddies' defence.
New signings Andrew Shinnie and Brian Graham both came off the bench to make their debuts for Hibs.
Graham in particular had a brilliant chance to score his first goal for the Leith side but Saints goalkeeper Scott Gallacher blocked the effort.
Former Buddie John McGinn cracked the angle of post and cross-bar late on and any critics of Cummings would say the forward should have gone home with a hat-trick ball.
His double here brought his tally for the season to five already, and the deflected cross from energetic captain Gray only needed a left-foot steer into the net.
Being in the right place to capitalise was key for Cummings, and 10 minutes later his persistence at pestering St Mirren defender Jack Baird and keeper Gallacher was rewarded.
The St Mirren duo left a back-pass to each other and Cummings nipped in to clinically score from a tight angle.
Lennon's managerial effect from the sidelines is clear. Fifteen minutes into this match the former Celtic boss was berating Hibs centre-back Darren McGregor for a lack of urgency.
Players paid attention to Lennon's demands to "move it quickly" and "get it forward".
Cummings received an earful for a poor second-half pass and Marvin Bartley was rebuked for moaning at the referee. Lennon has got his team by the scruff of the neck.
St Mirren's home form under Alex Rae has been better than his predecessors but there was not much for the locals to get excited about on this occasion - indeed some booed at half-time.
Baird watched his header sail over and substitute David Clarkson shot narrowly wide, but the impression was Hibs had another gear in them if required.
Hibs boss Neil Lennon: "Jason Cummings is a goalscorer but there's more to his game.
"He got two brilliant goals and could've had a couple more. I thought he got sloppy in the second half and looked a little bit tired.
"If you'd been watching him in the first half you could have had Real Madrid on the phone. But if you'd been watching him second half you would've had Accrington Stanley on the phone - and that's no disrespect to Accrington."
St Mirren manager Alex Rae: "The fact is that we dug in and competed in the second half.
"You can't legislate for that second goal. No matter what the preparation is, it's just a miscommunication and it becomes difficult because you're chasing the game.
"We need to stick together as a team and get through this."
Match ends, St. Mirren 0, Hibernian 2.
Second Half ends, St. Mirren 0, Hibernian 2.
Andy Webster (St. Mirren) wins a free kick in the defensive half.
Foul by Brian Graham (Hibernian).
John McGinn (Hibernian) hits the bar with a right footed shot from outside the box.
Attempt missed. Andrew Shinnie (Hibernian) right footed shot from a difficult angle on the right misses to the right.
Corner,  Hibernian. Conceded by Jason Naismith.
Attempt saved. Brian Graham (Hibernian) right footed shot from the centre of the box is saved in the top centre of the goal.
Jordan Stewart (St. Mirren) is shown the yellow card for a bad foul.
Foul by Jordan Stewart (St. Mirren).
Dylan McGeouch (Hibernian) wins a free kick in the attacking half.
Attempt missed. Stephen Mallan (St. Mirren) right footed shot from outside the box is high and wide to the right.
David Clarkson (St. Mirren) wins a free kick in the defensive half.
Foul by Darren McGregor (Hibernian).
Substitution, Hibernian. Martin Boyle replaces Jason Cummings.
Attempt missed. Brian Graham (Hibernian) right footed shot from the centre of the box misses to the right.
Attempt saved. David Clarkson (St. Mirren) right footed shot from the right side of the box is saved in the centre of the goal.
Attempt missed. Andrew Shinnie (Hibernian) right footed shot from outside the box is too high.
Foul by Kyle Hutton (St. Mirren).
Dylan McGeouch (Hibernian) wins a free kick in the attacking half.
Foul by David Clarkson (St. Mirren).
Marvin Bartley (Hibernian) wins a free kick on the left wing.
Foul by Gary Irvine (St. Mirren).
Jason Cummings (Hibernian) wins a free kick in the defensive half.
Substitution, Hibernian. Brian Graham replaces Grant Holt.
Substitution, St. Mirren. Jordan Stewart replaces Lawrence Shankland.
Attempt saved. David Gray (Hibernian) header from the right side of the box is saved in the top centre of the goal.
Andy Webster (St. Mirren) wins a free kick in the defensive half.
Foul by Andrew Shinnie (Hibernian).
Substitution, St. Mirren. David Clarkson replaces John Sutton.
Substitution, Hibernian. Andrew Shinnie replaces James Keatings.
Foul by Gary Irvine (St. Mirren).
Jason Cummings (Hibernian) wins a free kick in the defensive half.
Foul by Gary Irvine (St. Mirren).
Jason Cummings (Hibernian) wins a free kick on the right wing.
James Keatings (Hibernian) hits the bar with a left footed shot from outside the box.
Attempt missed. Jack Baird (St. Mirren) header from the centre of the box is close, but misses the top right corner.
Gary Irvine (St. Mirren) wins a free kick on the left wing.
Foul by David Gray (Hibernian).
Kyle Hutton (St. Mirren) is shown the yellow card for a bad foul.
Jason Cummings' brace against St Mirren helped Hibernian continue their perfect start to the Championship season.