A Rhodes Scholar, he studied political science and history from 1909 to 1911 while at Oxford University.

After the war, he received a PhD from the Sorbonne and later returned to the National Archives eventually becoming Dominion Archivist from 1937 to 1948.

A historian, he wrote many books including L'Administration de la Nouvelle-France (1929), Le Canada d'hier et d'aujourd'hui (1934), Montréal au temps de la Nouvelle-France, 1642-1760 (1942), Trois ans de guerre, 1939-1942 (1943), L'Oeuvre de la France en Amérique du Nord (1951), Histoire du Canada (winner of the 1963 Governor General's Award for French language non-fiction), Le Canada et la Révolution américaine (1965, and winner of the inaugural Albert B. Corey Prize in 1967)[1] and Montréal sous Maisonneuve, 1947-1965 (1966).

On July 6, 1967, he was one of the first people to be made an Officer of the Order of Canada (then called a Service Medal).

In late 2006 and early 2007, Lanctot made the news concerning his Order of Canada medal which was put up for auction on eBay.