Leo Gerard was born in Creighton Mine, Ontario on March 19, 1947, at the time an unincorporated suburb of Sudbury and a company town established by Inco. His father, Wilfred Gerard, was a miner at the Creighton Mine and a key organizer with the International Mine Mill and Smelters Workers’ Union (which merged with the United Steelworkers in 1967). He grew up in Sudbury. Taught that unions were supposed to be engaged on social issues and not just collective bargaining, Gerard often listened in on union meetings conducted in the family home. He handed out leaflets on the eve of a strike at the age of 11 and accompanied his father on a union organizing drive at the age of 13.

After graduating from Lively District Secondary School, 18-year old Gerard took a job at the Inco nickel smelter in Sudbury, unclogging tuyeres with a sledgehammer. He was elected steward and then chief steward of the 7,000-member Local 6500. He enrolled at Laurentian University part-time, studying economics at night and planning to be an economics professor. He quit college in 1977 when he was just a few credits short of graduation, and took a job as a staff representative for the international union.

He married his high school sweetheart, Susan, and they have two daughters.

Gerard rose steadily within the Steelworkers union hierarchy over the next two decades. After serving as chief steward for Local 6500, he was hired for a staff job with the union in 1977. He was elected director of USW District 6 in 1985 and re-elected in 1989, and was appointed national director of the Canadian division of the USW in August 1991. He was elected secretary-treasurer of the international union in 1993, and again in 1997. While USW secretary-treasurer, Gerard instituted a number of important administrative initiatives. He implemented cost-saving and revenue-generating initiatives, reorganized the secretary-treasurer's office, created an information technology department, developed a new union-to-member communications network, restructured member and local union servicing, and reinvigorated the union's organizing efforts.

Gerard eventually returned to Laurentian University and received a bachelor’s in economics and politics.  The university awarded him an honorary Doctor of laws degree in 1994.

Leo was appointed to the position of international president in February 2001 after the resignation of George Becker and went on to be elected in the following November election as well as two more 4 year terms in 2005 and 2009. Leo was the second Canadian to head the USW.