Kantor was born McKinlay but he added an "a" to his name because he thought it sounded more Scottish. His first name was Benjamin but while he was still a lad he decided he wanted to be called MacKinlay or just Mack. His mother was the editor of a small-town Iowa newspaper. His father abandoned the family before he was born.
Beginning in 1928, Kantor wrote stories for pulp magazines. His first novel, Diversey, was also published that year. His stories appeared in Real Detective Tales and Mystery Stories and in Detective Fiction Weekly. Kantor broke into the slick magazines in 1935, when one of his stories appeared in Collier's.
As a boy, Kantor had been fascinated listening to the stories of Civil War veterans. He began collecting first-hand narratives. During the 1930s, he wrote about the war. His novel Long Remember (1934) was set during the Battle of Gattysburg. Kantor served as a correspondent in London during World War II, where he interviewed wounded soldiers who told him their only goal was to get home alive. After the war, he wrote a novel in blank verse, Glory for Me (1945), about the returning soldiers. The title was taken from lyrics to a Protestant hymn. Later screenwriter Robert Sherwood wrote the script for The Best Years of Our Lives (1946) based on Kantor's book. The film won 13 Academy Awards, including one for Best Adapted Screenplay.
Kantor and his wife built a house on Siesta Key (off Sarasota) in 1937. At the time, Siesta Key had few residents and was largely a jungle. Kantor died in Sarasota in 1977. The MacKinlay Kantor Room exhibit at the Sarasota History Center reproduces his office from a photograph that appeared in Life magazine. It contains his desk, and his books, paintings, and photographs.