Francis Meli grabbed a brace of tries as St Helens claimed their first win in four matches to consolidate their place in the Super League top four.
Chris Flannery was the first to cross for Saints before Meli's tries extended their lead, both from close range.
Hull responded through Tom Briscoe, and Jordan Turner gave them further hope.
But Jamie Foster's penalty and Tommy Makinson's try ensured that Tom Briscoe's late effort was no more than a consolation for the visitors.
It was the end of a three-match winning streak for Hull but still a commendable performance, given the absence of Craig Fitzgibbon and late withdrawal of Sam Obst.
It was still a tough, physical encounter for Saints, in which big man Tony Puletua had to step up to the plate given the suspension of Louie McCarthy-Scarsbrook and the early loss of Josh Perry with a hamstring problem.
After Saints made their early pressure count when Flannery weaved over from close range, winger Meli then forced his way over twice in as many minutes.
Hull gave the large travelling contingent something to cheer when Cameron Phelps and Kirk Yeaman combined to send Tom Briscoe flying in at the corner.
Within five minutes of the restart, Hull centre Turner forced his way past opposite number Meli to ground the ball. But Danny Tickle again missed the conversion, and Foster kicked a 35-metre penalty to nudge Saints into a two-score lead.
Saints got the next crucial score when Sia Soliola sent Makinson away down the right to round off a great move.
Lomax put the icing on the cake when he dummied and went in from 20 metres. Briscoe finished off a long-range score on the hooter, goaled by Joe Westerman for a late Hull consolation.
POST-MATCH REACTION
St Helens coach Royce Simmons:
"We have come through a tough period from the Exiles game, with some tough games against Bradford, Wigan and Warrington. To get a win tonight was important and I have rewarded them with three days off.
"Tony Puletua was outstanding. When we lacked a bit of 'go forward' he took it on for us.
"I will settle for third or fourth now. If you would have asked me at the start of the year I would not have done. I was aiming for one or two but then we have had so many injuries and have had to use a lot of young blokes."
Hull coach Richard Agar:
"Overall our performance would have been good enough to get a win against a lot of sides. I was really happy with our attitude and commitment.
"In the first half we did not see the ball for six sets and Saints' execution in that period was outstanding.
"Likewise in the second half we did not see the ball for five sets and they scored twice. Their cutting edge on the night was a bit better than ours. They were pretty clinical."
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