Called to the Quebec Bar in 1962, Louis Lebel began his legal career in the field of labour law, practicing in Quebec City from 1963 to 1971 first at LeBel, Letarte, Bilodeau, Boily and then at Désilets, Grondin, LeBel & Associés. During this period, he earned a graduate degree in private law at Université Laval (1965) and a master's in law at the University of Toronto (1966). From 1971 to 1984, he was a partner at Grondin, LeBel, Poudrier, Isabel, Morin & Gagnon.
The author of numerous legal articles and studies and the co-author, with Robert-P. Gagnon and Pierre Verge, of Le droit du travail en vigueur au Québec, an examination of current labour law in Quebec, LeBel has served on various committees with the Barreau de Québec (city) and the Barreau du Québec, including the editing committee of the Revue du Barreau from 1976 to 1982, which he chaired from 1979 to 1982.
LeBel left private practice upon his appointment to the Quebec Court of Appeal in June 1984. He then served on the Supreme Court of Canada, from 2000 to 2014. The Université Laval granted him an honorary doctorate of law in 2001, and the University of Ottawa followed suit in 2010. He has been awarded the Barreau du Québec medal in 2000 and the Barreau de Québec (city) in 2008. He was made an honorary member of the American College of Trial Lawyers in 2004.
Louis LeBel was a member of the board of directors of the Corporation du Centre communautaire juridique de Québec from 1973 to 1975. Before that, he served on the Barreau du Québec's legal aid committee. He was the vice-president of the Barreau du Québec from 1982 to 1983 and its president from 1983 to 1984. He has taught as a guest professor at the University of Ottawa and the Université Laval.
He joined Langlois lawyers as a legal advisor in May 2015, after retiring from the Supreme Court. He was appointed a mentor of the Pierre Elliott Trudeau Foundation in 2016.