His success there brought him to the attention of The Saturday Evening Post in New York City where he was fiction editor for fourteen years. Beginning in 1914, he was a staff writer for and, from 1917, editor of Toronto-based Maclean's magazine. Also in 1910, Costain joined the Maclean Publishing Group where he edited three trade journals.

He married Ida Randolph Spragge (1888–1975) in York, Ontario on January 12, 1910. He was an editor at the Guelph Daily Mercury between 19. His first writing success came in 1902 when the Brantford Courier accepted a mystery story from him, and he became a reporter there (for five dollars a week). These early novels were rejected by publishers. Before graduating from high school he had written four novels, one of which was a 70,000 word romance about Maurice of Nassau, Prince of Orange. He attended high school there at the Brantford Collegiate Institute. Henry Darlington Steinmetz Costain was born in Brantford, Ontario to John Herbert Costain and Mary Schultz. Costain was born in Brantford, Ontario to John Herbert Costain and Mary Schultz.

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