Born Lewis Frederick Ayre III in Minneapolis, Minnesota and raised in San Diego, California, Ayres began acting in bit player roles in films in 1927. He played opposite Greta Garbo in 1927's The Kiss, but it was his starring role in 1930's All Quiet on the Western Front which made him a star. He played the title role in Young Dr. Kildare in 1938, and became a matinee idol, starring in several Kildare films.

But his conscientious objector status during World War II caused outrage throughout America, until he volunteered with the Medical Corps, serving in the Pacific and in New Guinea. In 1948 he was nominated for the Academy Award for Best Actor for Johnny Belinda, but his career was sparse after the war. Fatefully his Johnny Belinda costar, Jane Wyman fell in love with Ayres and left her husband, Ronald Reagan, for him, albeit unsuccessfully. Had she not done so, however, the history of the world in the 20th century would be quite different.

He was offered the part of Doctor Kildare in a television series, but his (now prescient) request that the show not have cigarette ads torpedoed that.

His 1976 documentary film Altars of the World brought his Eastern philosophical beliefs to the screen and earned him critical acclaim.

Late in life, he appeared on The Mary Tyler Moore Show as the father of the Murray Slaughter character played by Gavin MacLeod. The episode involved Mary's May-November romance with Mr. Slaughter, Senior.

Ayres was married three times. He was married to actress Lola Lane from 1931 until 1933 and to actress Ginger Rogers from 1934 until 1940. His third marriage, to Diana Hall, lasted from 1964 until his death from undisclosed causes at the age of 88.

He has a star on the Hollywood Walk of Fame at 6385 Hollywood Blvd.