Gaines was born in Indian Territory and grew up in Texas, learning "to handle the ax, the plough, and the lariat".

[1] He worked at a variety of jobs in the United States and in France before winning a scholarship to study at the American Laboratory Theatre.

Gaines made his film debut as Patrick Henry in the historical drama The Howards of Virginia with Cary Grant.

One of his best-known roles was as Jean Arthur's stuffy suitor Charles J. Pendergast in The More the Merrier (1943), directed by George Stevens.

He also appeared as the insurance company boss of Fred MacMurray and Edward G. Robinson in the film noir classic Double Indemnity (1944).