What was the first movie based on a TV series?

The first movie based on a TV series was Dragnet (1954).

It was based on the NBC series of the same name (NBC, 1952-59, 1967-70).

What is John McLaughlin’s sign-off line on “The McLaughlin Group” (synd., 1982)? John McLaughlin's sign-off line on "The McLaughlin Group" was "Bye-bye."
What was the Flying Nun’s name on the TV series “The Flying Nun,” (ABC, 1967-70)? Sister Bertrille, born Elsie Ethington (played by Sally Field) was the Flying Nun's name on the TV series "The Flying Nun." Convent San Tanco, in Puerto Rico, was the name of her convent.
Who plays Percy Shelley and Lord Byron in The Bride of Frankenstein (1935)? In the prologue that frames the story, Douglas Walton plays Shelley and Gavin Gordon plays Lord Byron in The Bride of Frankenstein. Elsa Lanchester plays Mary Shelley (author of the novel Frankenstein), as…
On what book was A Place in the Sun (1951) based? A Place in the Sun is based on the book An American Tragedy, by Theodore Dreiser.
Who wrote Shane, the book on which the 1953 movie is based? Jack Schaefer wrote the book Shane, which was also movie.
On what work was Dr. Strangelove (1964) based? Dr. Strangelove was based on a novel, Red Alert, by Peter George.