[3] Lemieux studied at the local Saint-François-de-Laval school, in the Ahuntsic district of Montréal, but did not excel in classes, spending most of time on the streets.
Encouraged by his adoptive father to focus, he started training in Ring 83 located in Ahuntsic at the invitation of boxer Baha Laham, a neighbour of Lemieux.
Instead, he and Russ Anber viewed the Olympic program as a dead-end which would have him competing against boxers with vastly more experience while limiting his training prospects in ways detrimental to a fighter with professional aspirations.
On March 11, 2011, it was announced that Lemieux would appear in a WBC final eliminator against Marco Antonio Rubio (49–5–1, 43 KOs) at the Bell Centre in Montreal, Quebec, on April 8 in the main event of ESPN's Friday Night Fights.
After the fight Anber said that Lemieux still has a lot to work and improve on if he wants to reach the top level.
At the same time, his promoter Groupe Yvon Michel, announced that he would next fight former WBA light middleweight champion Joachim Alcine (32–2–1, 19 KOs) on December 10 at the Bell Centre in Montreal.
[18][19] His next bout was confirmed on October 20, 2014, to take place Barclays Center in Brooklyn against fringe contender Gabriel Rosado (21–8, 13 KOs), defending his NABF middleweight belt on December 6.
It was the first HBO televised bout for Lemieux, who was hoping to challenge unified middleweight champion Gennady Golovkin in the near future.
[25] In February 2015, Lemieux accepted a fight against former interim WBO titleholder Hassan N'Dam N'Jikam (31–1, 18 KOs) for the vacant IBF middleweight championship.
The IBF then ordered British contender Billy Joe Saunders, however he also passed as he was already mandated to fight for the WBO title.
[27] On May 13, Golden Boy and Eye of the Tiger Management announced the fight would take place at the Bell Centre in Montreal on June 20.
HBO, who has recently signed Lemieux were unable to squeeze the bout into their boxing schedule for unknown reasons.
It was announced in July 2015 that Gennady Golovkin would be defending his three world titles against Lemieux in a unification fight at the Madison Square Garden in New York City on October 17, 2015, live by HBO Pay-Per-View.
Lemieux won the then vacant IBF title by outpointing Hassan N'Dam N'Jikam in June 2015.
[35] Lemieux lost the IBF middleweight title to Golovkin before a sold-out crowd of 20,548 at Madison Square Garden.
His opponent was announced to be Glen Tapia (23–2, 15 KOs), who was newly signed to Golden Boy and moving up in weight.
[41][42] Lemieux was taken the distance only the fourth time in his career as he won a one-sided 10 round unanimous decision.
The win ensured Lemieux a possible future fight against the likes of Saul Alvarez or WBO middleweight champion Billy Joe Saunders in early 2017.
[43] On January 16, 2017, a fight between Lemieux and former world title challenger Curtis Stevens (29–5, 21 KOs) was finally agreed to main event a HBO: Boxing After Dark card on March 11 at the Turning Stone Resort & Casino in Verona, New York.
The referee Charlie Fitch waived the count immediately after Stevens was left unconscious The stoppage took place in round 3, at 1 minute and 59 seconds.
Reyes showed great determination and stayed on his feet landing quick combinations, although receiving a cut above the right eye in round 2.
[55][56] Lemieux made himself available as Miguel Cotto's final opponent, which would take place on December 2, 2017, on HBO at Madison Square Garden.
[59] On October 16, Lemieux's manager Camille Estephan stated the negotiations were going well for the fight and could possibly take place at the Bell Centre in Canada on December 16, 2017.
[61] Lemieux failed in his attempt to regain a world title after losing to Saunders via a wide unanimous decision.
He was given time to reduce his weight further, however within an hour, Camille Estephan told the officials that Lemieux would not be making the middleweight limit.
[70] In July 2018, Estephan began negotiating a deal for Lemieux to fight Irish boxer Gary O'Sullivan (28–2, 21 KOs) in the fall.
[71] After winning defeating Elias Espadas on July 21, Olympic medalist Yamaguchi Falcão, (16–0, 7 KOs) also made it known that he would fight Lemieux in September 2018.
[72] On August 1, via Irishboxing.com, O'Sulivan accepted the challenge and signed the contract a week later, awaiting confirmation of purse information.
[78] On October 31, 2018, it was first reported that Lemieux would appear on the Canelo Álvarez vs. Rocky Fielding undercard at Madison Square Garden on December 15, 2018.
Lemieux earned the respect of many in this match-up, showing a big heart when he continued to fight even though he received hard punches early in the first few rounds that had him dazed.