Glennon was born in Anaconda, Montana in 1895 and attended Stanford University, where he graduated in 1912. Before gaining fame in Hollywood, Glennon served as a pursuit pilot instructor during World War I. He began his work in film in 1912 as a stage manager for theater entrepreneur Oliver Morosco and then c. 1913 worked for Keystone and Famous Players, then was laboratory superintendent for Clune Film Corporation, for four years. In 1915 he did his first film as cinematographer The Stingaree (serial) and in 1928 he directed his first film The Perfect Crime.