Blackmer was born and raised in Salisbury, North Carolina, the son of Clara Deroulhac (née Alderman) and Walter Steele Blackmer. He started off in an insurance and financial business but gave up on it. While working as a builder's laborer on a new building, he saw a Pearl White serial being filmed and immediately decided to go into acting. He attended the University of North Carolina at Chapel Hill. Blackmer went to New York, hoping to act on the stage. While in the city, he took jobs and extra work at various film studios at the then motion picture capital, Fort Lee, New Jersey, including a bit part in the highly popular serial, The Perils of Pauline (1914).
He made his Broadway debut in 1917, but his career was interrupted by service in the U.S. military in World War I. After the war, he returned to the theatre and, in 1929, returned to motion pictures and went on to be a major character actor in more than 120 films. He won the 1950 Tony Award for Best Actor (Drama) for his role in the Broadway play, Come Back, Little Sheba.