He appeared in a wide variety of roles, such as the weak-willed prisoner Kent in The Big House (1930), the psychotic Danny in Night Must Fall (1937), and Joe, the boxer mistakenly sent to Heaven in Here Comes Mr. Jordan (1941).

[4][5] His father was president of the New York Rubber Company, and died by suicide in 1922 by jumping off the Brooklyn Bridge, when the family's fortune was gone.

He established a stage career, and became popular enough to turn down an offer to appear opposite Vilma Bánky in the film This Is Heaven (1929).

Norma Shearer chose him to star opposite her in The Divorcee (1930), Strangers May Kiss (1931), and Private Lives (1931), which led him to stardom.

During this time, Montgomery appeared in the original pre-Code film version of When Ladies Meet (1933), which starred Ann Harding and Myrna Loy.

Montgomery played a psychopathic murderer in the thriller Night Must Fall (1937), for which he was nominated for the Academy Award for Best Actor.

After World War II began in Europe in September 1939, and while the United States was still officially neutral, Montgomery enlisted in London for the American Field Service and drove ambulances in France until the Dunkirk evacuation.

[9] Montgomery returned to playing light comedy roles, such as Alfred Hitchcock's Mr. & Mrs. Smith (1941) with Carole Lombard.

[4][16] The couple had three children: Martha Bryan, who died at 14 months of age in 1931; Elizabeth (April 15, 1933 – May 18, 1995), an actress best known for her 1960s television series, Bewitched; and Robert, Jr., (January 6, 1936 – February 7, 2000).

His second wife was Elizabeth "Buffy" Grant Harkness (1909–2003), wed on December 9, 1950, four days after his divorce from Allen was finalized.