[3] Donaldson began his acting career in December 1937 when he appeared in an NBC radio show.

[3] In 1941, he played Tiny Tim in a week-long serialized version of Dickens' A Christmas Carol that was presented on Wheatena Playhouse.

[2] The performance led to a starring role as Arthur "Pinky" Thompson in his first movie, Once Upon a Time (1944), opposite Cary Grant and Janet Blair.

In 1945, Donaldson was cast in A Tree Grows in Brooklyn, which marked the directorial debut of Elia Kazan.

[2] Donaldson also starred as Danny Mitchell in the 1940s Rusty series of eight films about a German Shepherd dog.