Garnett Gladwin Sedgewick was born May 20, 1882, in Middle Musquodoboit, Nova Scotia, to Henry A. and Bessie Woolery (née Gladwin) Sedgewick. He attended high school while living with relatives in Oxford, Nova Scotia and then taught grade school (1900/01) in Oyster Pond, Jeddare, Nova Scotia. He then attended Dalhousie University in Halifax, graduating with a BA in 1903 (Honours in Classics and English). Sedgewick served as principal of schools in Oxford, Nova Scotia (1903-1905) and Nanaimo High School (1905-1907), History Master at St. Andrew's College in Toronto (1907-1908), and high school teacher in Vancouver (1908-1910). Sedgewick received his MA from Harvard University in 1911 and Ph.D. from Harvard in 1913. He was an instructor and assistant professor at Washington University in St. Louis from 1913 to 1918 when he became an associate professor and acting head of the Department of English at UBC. In 1920, he was made a professor and first head of the department. In 1934 he was the Alexander Lecturer at the University of Toronto (these lectures were later published), and in 1946 he was elected a Fellow of the Royal Society of Canada. He retired from UBC in 1948 and the same year was awarded an honorary LLD from Dalhousie.

Sedgewick was known for his lectures on Shakespeare and Chaucer; he also wrote scholarly articles, radio broadcasts and a weekly column, "More Heat than Light" for the Vancouver Sun. He served on the University Senate and was involved with the Vancouver Art Gallery, Symphony and Little Theatre, as well as the Civil Liberties Union. The former undergraduate library at UBC was named in his honour, as were the Sedgewick Lectures, sponsored by the Department of English. Sedgewick died in Vancouver in September 1949.