His early appearances included "Webs of Steel" (1925), "Lorraine of the Lions" (1925), and "The Calgary Stampede" (1925), a Hoot Gibson Western. Brennan was also in "Watch Your Wife" (1926), "The Ice Flood" (1926), "Spangles" (1926), "The Collegians" (1926, a short), "Flashing Oars" (1926, a short), "Sensation Seekers" (1927), "Tearin' Into Trouble" (1927), "The Ridin' Rowdy" (1927), "Alias the Deacon" (1927), "Blake of Scotland Yard" (1927) (a serial), "Hot Heels" (1927), "Painting the Town" (1928), and "The Ballyhoo Buster" (1928). The latter was directed by Richard Thorpe who would use Brennan as an extra several times on films.