Keeshan also played the original "Clarabell the Clown" on the Howdy Doody television program.

Keeshan was born in Lynbrook, NY. In 1945, during World War II, he enlisted in the United States Marine Corps Reserve, but was still in the United States when Japan surrendered. He attended Fordham University on the GI Bill. An urban legend claims that actor Lee Marvin told on "The Tonight Show" how he had fought alongside Keeshan at the Battle of Iwo Jima in February-March, 1945. However, Marvin not only never said this, but had not served on Iwo Jima (having been hospitalized from June, 1944, until October, 1945, from wounds received in the Battle of Saipan), and Keeshan himself never saw combat, having enlisted too late to serve overseas.

Network television programs began shortly after the end of the Second World War. Howdy Doody, an early show which premiered in 1947 on NBC, was one of the first. Debuting on January 3, 1948, Keeshan played "Clarabell the Clown", a silent Auguste clown who communicated by honking several horns attached to a belt around his waist. One horn meant "yes"; another meant "no". Clarabell often sprayed Buffalo Bob Smith with a seltzer bottle and played practical jokes. Keeshan gave up the role in 1952, and was replaced.

By August 1953, Keeshan was back on the air on WABC-TV (New York City), in a new children’s show, Time for Fun. He played Corny the Clown, and this time he spoke. Later that same year, in addition to Time for Fun, Keeshan began Tinker's Workshop, a program aimed at preschoolers, with him playing the grandfather-like Tinker.

Developing ideas from Tinker's Workshop, Keeshan and his long-time friend Jack Miller submitted the concept of Captain Kangaroo to the CBS network, which was looking for innovative approaches to children's television programming. CBS approved the show, and Keeshan starred as the title character when it premiered on CBS on October 3, 1955. Keeshan described his character as based on "the warm relationship between grandparents and children." The show was an immediate success, and he served as its host for nearly three decades. The show ran until 1984.

Recurring characters included Mr. Green Jeans (played by Hugh "Lumpy" Brannum) and puppets such as "Bunny Rabbit" and "Mr. Moose".

The New York Times commented: "Captain Kangaroo, a round-faced, pleasant, mustachioed man possessed of an unshakable calm ... was one of the most enduring characters television ever produced."

Keeshan also had a Saturday morning show called Mister Mayor during the 1964-65 season. Keeshan, in his role as the central character in both Captain Kangaroo and Mister Mayor, heavily promoted the products of the Schwinn Bicycle Co., a sponsor, directly on-air to his audience. By 1972, Keeshan had introduced another character on Captain Kangaroo to recommend Schwinn products, Mr. Schwinn Dealer, due to the Federal Trade Commission ruling against children's show hosts directly endorsing their sponsor's products during their programs after 1969.

Keeshan suffered a severe heart attack on July 13, 1981 , which pushed the start of a revamped version of his show back to at least mid-August. Keeshan suffered the heart attack just moments after stepping off a plane at Toronto International Airport. He had come to the city to accept a children's service award. It was said to be the beginning of the end of Keeshan's career as the captain.

Keeshan underwent triple-bypass surgery and received an estimated 5,000 get-well wishes from children whose lives he had touched over the years .