Two goals in three minutes, from Rowan Vine and Dave Mackay, made the difference midway through the second half as Saints leapfrogged Aberdeen into fifth place in the Clydesdale Bank Premier League.
Gregory Tade had opened the scoring with his second one-on-one in two minutes but Niall McGinn levelled for the Dons just before the break.
Lomas said: "It was a great win and well deserved. The lads responded very well and I thought it was a comfortable win.
"Gregory did so well with the opener because he had just missed a chance. He could have been forgiven for snatching at the second chance but he was very composed."
Aberdeen manager Craig Brown could not explain his side's poor defending.
A poor touch from Isaac Osbourne set up Tade for the opener and Vine scored after Jamie Langfield failed to collect a corner, with two Aberdeen players unable to head to safety.
Callum Davidson then capitalised on some hesitancy before Steven MacLean cut back for Mackay to net.
Brown said: "It was a poor performance. How we can go from being excellent at the weekend to playing poorly in the last period of the game was unaccountable.
"Each goal was a gift. The first goal we conceded was alarmingly softly but St Johnstone took their chances."
- Sports & Recreation
- Gregory Tade
- Rowan Vine