Lemieux was the eldest of six children in a Radio-Canada technician's family. He was educated at College Mont St. Louis in Montreal, and in 1965 obtained his law degree from McGill University, where he was influenced by civil libertarian Frank Scott.
After passing his bar exams, he went to work for O'Brien, Home, Hall, Nolan, Saunders O'Brien and Smyth.
Lemieux was a highly emotional, volatile showman who was himself jailed for four months during the 1970 FLQ crisis under the War Measures Act on charges of seditious conspiracy. During his career as a lawyer for the FLC, Lemieux defended more than 30 terrorists and represented the members of the "Chenier cell", the group behind the kidnapping of Quebec cabinet minister Pierre Laporte, who was strangled.
In 1966 he was asked to represent Robert Levesque, an early FLQ member who faced six charges of robbing and bombing an armoury. While legal wrangling over the case continued, Levesque spent two years in jail without trial. Though Levesque was eventually sentenced to seven years in prison, Lemieux was angered by the delays. He was fired from the law firm and started taking other politically charged cases, defending FLQ terrorists Pierre Vallieres and Charles Gagnon, who were charged in 1966 with placing bombs in the LaGrenade shoe factory that killed a woman. In 1968, Lemieux moved his law practice into a room in the Nelson Hotel in Old Montreal.
Lemieux argued in court that Laporte's death was accidental and suggested prime minister Pierre Trudeau was partly responsible.
"If Trudeau had not declared the War Measures Act, Pierre Laporte would never have died," Lemieux said. Arrests under the War Measures Act, he raged, were "a shameful game, nameless buffoonery and extraordinary farce."
He defended many of those arrested under the act, as well as Laporte's kidnappers, Jacques and Paul Rose, Francis Simard and Bernard Lortie. He later negotiated their exile to Cuba.
By the late 1970s, Lemieux was a pariah in Montreal legal circles, and took his law practice to Quebec's North Shore, where he continued to be, in his words, "a ferocious Quebec separatist."
Lemieux supported himself taking on union grievances and aboriginal claims"