He began his career as a standup comedian and played the title character of the popular early U.S. television series Mister Peepers from 1952 to 1955.

[1] When he was 10, he moved with his divorced mother (mystery author Eleanor Blake) and a younger sister to Evanston, Illinois, where he became close friends with another child in the neighborhood, Marlon Brando.

According to the accounts of a fellow enlisted soldier, Cox adopted odd behaviors while undergoing basic training at Camp Wolters, Texas, such as putting on a uniform and full pack to pick flowers on Sundays, to receive a discharge from the Army.

Just as the studio audience had reached a peak of laughter, Cox suddenly switched gears, changed characters, and sang a high-pitched version of "The Drunkard Song" ("There is a Tavern in the Town"), punctuated by eccentric yodels.

"Wallace Cox" earned a big hand that night, but lost by a narrow margin to The Chordettes; yet he made enough of a hit to record his radio routine for an RCA Victor single.

Series producer Fred Coe approached Cox about a starring role in a proposed live television sitcom Mister Peepers, which he accepted.

He played a prominent supporting role as Preacher Goodman in Spencer's Mountain (1963), a Navy sonar operator in The Bedford Incident (1964), and a drug-addicted doctor opposite Marlon Brando in the World War II suspense film Morituri (1965).

He played character roles in more than 20 motion pictures and worked frequently as a guest star in television drama, comedy and variety series in the 1960s and early 1970s.

He was cast as a down-on-his-luck prospector seeking a better life for his family in an episode of Alias Smith and Jones, a Western comedy; and in Up Your Teddy Bear (aka Mother) (1970), he starred with Julie Newmar.

His television and screen persona was that of a shy, timid but kind man who wore thick eyeglasses and spoke in a pedantic, high-pitched voice.

Cox wrote a number of books, including Mister Peepers: A Sort of Novel, co-written with William Redfield,[10] which was created by adapting several scripts from the television series; My Life as a Small Boy, an idealized depiction of his childhood; a parody and update of Horatio Alger in Ralph Makes Good, which was probably originally a screen treatment for an unmade film intended to star Cox; and a children's book, The Tenth Life of Osiris Oakes.

While he maintained a meek onscreen persona, TV viewers did get a glimpse of Cox's physicality on an episode of I've Got a Secret, aired on May 11, 1960, in which he and host Garry Moore ran around the stage assembling furniture while the panel was blindfolded.