He studied at the University of Toronto, Osgoode Hall law school, and Harvard University.
He taught law at the University of Toronto for more than twenty years, first as a lecturer from 1940 to 1943, then as assistant professor from 1943 to 1945 and then, after a four-year period as lecturer at Osgoode, as professor of law from 1949 to 1965.
In 1965 Laskin was appointed justice of the Ontario Court of Appeals. Five years later he became the first Jewish judge of the Supreme Court of Canada. He was also the first purely academic lawyer and the youngest man appointed to the highest bench.
In 1973 he was named chief justice. Laskin also served as chancellor of Lakeside University in Thunder Bay, as president of the Association of Canadian Law Teachers from 1953 to 1954, as president of the Canadian Association of University Teachers from 1964 to 1965, and as chairman of the Legal Committee of the Canadian Jewish Congress and the Toronto Friends of the Hebrew University.