Aberdeen started well, with Andreas Mayer coming very close to opening the scoring, however Saints keeper Main saved well.
New signing Thomas Solberg nearly made a dream debut when he came close with a header, but it wasn't to be his, or the Dons day.
The visitors opened the scoring after 26 when Kevin Thomas was put clean through on goal, and he finished convincingly.
Gary Smith almost scored Aberdeen's first league goal of the season, but his header couldn't find the net.
Into the second half, and St Johnstone began to control the match. Gary Bollan came close to scoring a spectacular free-kick, but his thunderbolt shot was well saved by Dons goalie Dave Preece.
Preece was then blamed for the saints second goal, when Jim Weir struck to make it 2-0.
Nathan Lowndes completed the scoring after some bad defensive play from new don Solberg.
Final score: Aberdeen 0-3 St Johnstone