The Englishman is now eyeing a unification fight with the Kazakhstani fighter after racking up 25 straight victories before making his first appearance in North America on Saturday.
The 28-year-old, who has already defended the belt twice since winning it from Ireland's Andy Lee in 2015, was a convincing victor on all the judges' scorecards - 120-108, 117-111, 118-110 - and remains undefeated. Saunders doesn't have a big punch, but he showed himself to be a stylish fighter potentially capable of creating problems for anyone in his weight class, and that includes the two big guns, Gennady Golovkin and Canelo Alvarez.
'He won't be landing with none of them - he'll be missing with a load of them and then I'll be coming back and I'll break his heart'.
Saunders got the upper hand from the opening bell as he peppered the Canadian with jabs and combinations, winning every round on one judge's scorecard.
Saunders said: 'I think he's probably been in with a lot bigger punchers than me, a lot bigger, but he hasn't been in the ring with someone as slick as me. I look easy to put down.
And Saunders' trainer Dominic Ingle was incensed as he thought Lemieux was allowed to jump off the scales before the weight had settled. But you can't hit me.
Without him, "my boxing career would be finished", he said.
The WBO middleweight title fight tonight at the Bell Center in Laval, Quebec, Canada, was a rare specimen, a true "pick-'em" contest on the books of United Kingdom bookmakers.
By the fifth, Saunders began taunting his opponent for his misses, drawing boos from the crowd, but went on to land the better shots.
Lemieux, meanwhile, claimed to have injured his left hand in the second round, but lacked humility - and indeed a grasp on reality - when he refused to credit Saunders for his victory. He was on the run from the first round to the end.
Saunders (26-0) made a third defense of the title the British fighter won from Andy Lee in 2015 and showed he could be a force in the division that is now ruled by stars Canelo Alvarez and Gennady Golovkin. "Fight me now and you'll be punching fresh air".