It was Saints' first win over United in 13 years and keeps them one point behind third-placed Inverness with two SPL games remaining.
Liam Craig's first-half strike at Tannadice earned them the points in an otherwise dull game, with Lomas still dreaming of Europe.
He said: "We are still in there fighting. We just wanted to take into the last game.
"We have a big game against Celtic next week. We know it will be difficult. There are a few walking wounded in there.
"But it is no coincidence that we had a tremendous away support, they played a big part today. For the players to come out and see 1500 away fans makes a big difference.
"I thought we deserved the three points, especially after the first 15 minutes. Our keeper didn't have a save to make."
Saints midfielder Craig- who has signed a pre-contract with Hibernian - scored on 38 minutes with a fine finish from inside the penalty area.
Craig said: "I want to leave on a high but it (Europa League) is out of our hands, all we can do is keep winning games.
"If Inverness hang on to third place then they deserve it."