Hartnell entered the theatre in 1925 working under Frank Benson as a general stagehand. He appeared in numerous Shakespearian plays, including "The Merchant of Venice" (1926), "Julius Caesar" (1926), "As You Like It" (1926), "Hamlet" (1926), "The Tempest" (1926) and "Macbeth" (1926). He also appeared in "She Stoops to Conquer" (1926), "The School for Scandal" (1926) and "Good Morning, Bill" (1927), before performing in "Miss Elizabeth's Prisoner" (1928). This play was written by Robert Neilson Stephens and E. Lyall Swete. It featured the actress Heather McIntyre, whom he married during the following year. His first of more than 60 film appearances was in "Say It With Music" (1932).