While Aberdeen were buoyed by the return of Bojan Miovski, who only made one substitute appearance during the League Cup group stage, St Johnstone boss Craig Levein admitted he was only "reasonably happy" with how the summer has gone.
He would have been happier had Lewis Neilson hit the target with his header from three yards out. Instead, the on-loan Hearts defender put it wide.
Aberdeen fired a warning shot when Ester Sokler got in behind, rounded the keeper and tucked home. The flag went up to save St Johnstone, but they were under the cosh.
Miovski had a shot blocked well in the box before he rattled the crossbar from 25 yards. Sokler had one saved at close range by Josh Rae.
A minute later, the Dons hit the front. Devlin was the man to make the breakthrough, rising high to glance home a header.
St Johnstone did create a few chances, with Benjamin Kimpioka the man on the end of most of them. His dipping half-volley flew just over the bar.
The Dons' second goal was all about the assist from Sivert Heltne Nilsen, a surgical pass that bisected the defence and presented a golden chance to McGrath, with the unmarked Irishman sliding the ball in.
McGrath is canny at creating, too. He whipped in a delightful cross that new signing Molloy almost headed in, with the ball drifting just the wrong side of the post.
There was time for a nervy ending. Makenzie Kirk - son of Saints assistant Andy - was introduced and immediately blasted in a low shot that cannoned off Molloy and past a helpless Dimitar Mitov.
The young striker then had hearts in mouths when he got on the end of Matt Smith's fantastic link-up, but could only blast straight at Mitov.