Born Wallace Maynard Cox in Detroit, Michigan, he sprang into the nation's consciousness when he was cast as school teacher "Robinson Peepers" in the 1952 TV series Mr. Peepers. The show debuted as a summer replacement, but moved to the regular line-up after garnering strong reviews and ratings.

Cox, also starred as "Hiram Holiday" in the 1956 series The Adventures of Hiram Holiday and provided the voice of "Underdog" for the cartoon show of the same name, famous for the quote, "There's no need to fear! Underdog is here!"

No more odd couple could be imagined than the mousy Wally Cox and his life-long friend and one-time roommate Marlon Brando. The two roomed together when Brando came to New York to study with Stella Adler, but Cox ultimately moved out because he could no longer stand Brando's pet raccoon. A frequent TV guest star, Cox's memorable performances included playing computer operator "James Elwood" in The Twilight Zone episode: "From Agnes - with Love".

Once Cox appeared in a TV commercial for men's underwear in the 1960s, but Standards and Practices at the time banned the commercial from broadcast. The offending material was Wally opening his dress shirt to reveal a small segment of his undershirt. This was considered "exposing underwear on a live model" and the commercial never aired.

Cox wrote a mystery novel for children, The Tenth Life of Osiris Oakes, about a young boy who steals a mummified cat from a museum, only to have it come back to life. He died in Los Angeles, California, of a heart attack.