Hull ran in seven tries at the KC Stadium as they eased to their first Super League win in a month.
Jordan Turner, Willie Manu, Sam Obst and Tom Briscoe all crossed before the break.
Richard Horne, Sam Moa and Richard Whiting then ran in three more in the second half, Chris Melling grabbing a consolation for hapless Harlequins.
Quins, who have now lost their last seven games, are without a Super League win since 20 March.
Their torrid run of form continued as Hull's four internationals were able to back up from Friday's England-Exiles clash with relative ease.
Exiles trio Craig Fitzgibbon, Willie Manu and Mark O'Meley, and England wing Tom Briscoe, were given the tamest tests of their fitness levels.
Manu and Briscoe both scored as well, but it was another man once tipped for international honours who stole the show for Richard Agar's men.
Joe Westerman was once hailed as the next superstar of the British game, only to be overtaken by the likes of Sam and Joel Tomkins.
While they are now both established internationals, Westerman has been forced to settle for a place in the England Knights squad.
But he has been gradually improving since swapping Castleford for Hull and was outstanding against Quins, leading their attack in the absence of injured scrum-half Sean Long.
Quins did not have a single player on representative duty and, in theory, should have been fresher than their hosts.
But they were on the back foot from the moment that former Hull winger Mark Calderwood put the ball down on his own 30-metre line, his old club taking full advantage of his error by going in for a sixth-minute opener.
Westerman's delightful delayed pass put Turner over, Danny Tickle adding a simple conversion, his 400th goal for the club.
Calderwood was not enjoying the sort of return he would have wished for when, after 11 minutes, he and loan full-back Chris Riley got in a tangle as they looked to see Richard Horne's kick out, only to both miss the ball and allow Manu to follow his Exiles try with a second four-pointer of the weekend.
For all their dominance, Hull did not look particularly menacing, especially after Horne left the field with a cut, and it was left to Westerman to orchestrate their attacks.
It was from a move started by him that they scored again through Obst nine minutes before the half-time hooter, Westerman himself adding the extras.
And he then created the space for Kirk Yeaman to lay on Briscoe's 12th try of the season.
Seven minutes after the break, Westerman was on song again, breaking 40 metres to lay on a play that eventually saw Manu put the returning Horne over, Moa following him four minutes later after another Manu pass.
Chris Melling did get the Quins on the scoresheet with 22 minutes remaining before Hull hooker Danny Houghton was carried off wearing a neck brace following a lengthy delay.
But Whiting went through a gap after a neat left-side move to complete the scoring, Tickle adding his fourth conversion.
POST-MATCH VERDICT
Hull coach Richard Agar:
"I think England need Joe Westerman. I'm not just talking about England the team, but as a game we need people like Joe coming through.
"He cares a great deal about his performances and I think it's good that our club have given him a challenge in the back row.
"He's probably not had as much game time as he'd like but sometimes his performances have been a little bit loose and not warranted it.
"That has made him aim up in a couple of areas of his game which he's started to do, and, having moved to a new club, he is starting to find his role in the team."
Harlequins coach Rob Powell:
"We weren't very good. We completed about 62% which isn't enough to compete in the game.
"The players are working hard and training with a lot of intensity but it wasn't always there today.
"We've got to keep giving them the confidence they need, make training competitive and make sure they're ready to go each week."
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