In 1935, Cook's starring role in the film The Spanish Cape Mystery made him the first actor in any medium to play fictional sleuth Ellery Queen.

He played the role of Steve opposite Helen Morgan's Julie in the 1936 film adaptation of Show Boat, was one of the suspects in the Philo Vance mystery The Casino Murder Case, and starred as an heroic U.S.

During his 1930 summer stock engagement at Elitch Theatre, Cook met and fell in love with Frances Beranger, another member of the company.

However, “the prestige of my Elitch engagement helped me get my first Hollywood contract – with Warner Brothers.”[8] Cook was married to Princess[9] Gioia Tasca di Cuto,[10] from 1937 until his death in 1961 (although they were legally separated at the time)[4] from a heart attack in New Haven, Connecticut, five days after his 60th birthday, in the midst of rehearsals for Cook's new play, A Shot in the Dark, an adaptation of L'Idiote.

[12] For his contributions to the motion picture industry, Cook was honored with a Hollywood Walk of Fame star located at 1718 Vine Street.