Sorry IO, but I think your leg was pulled! According to snopes.com and some other bio sites, he enlisted in the Marines but never saw combat.
"Bob Keeshan, later famous as television's "Captain Kangaroo," also enlisted in the U.S. Marines, but too late to see any action during World War II. Keeshan was born on 27 June 1927 and enlisted two weeks before his 18th birthday, months too late to have taken part in the fighting at Iwo Jima. A 1997 interview with Keeshan noted that he "later enlisted in the U.S. Marines but saw no combat" because, as Keeshan said, he signed up "just before we dropped the atom bomb."
His biography according to the Museum of Broadcast Communications:
BOB KEESHAN. Born in New York City, New York, U.S.A., 27 June 1927. Attended Fordham University, 1946-49. Served in United States Marine Corps Reserve, 1945-46. Married: Anne Jeanne Laurie, 1950; children: Michael Derek, Laurie Margaret, and Maeve Jeanne. Began career as Clarabell for NBC-TV's Howdy Doody Show, 1947-52; appeared as Corny the Clown (ABC-TV), 1953-55, and Tinker the Toymaker (ABC-TV), 1954-55; starred as Captain Kangaroo (CBS-TV), 1955-85; president of Robert Keeshan Associates, from 1955; appeared as Mr. Mayor and the Town Clown (CBS-TV), 1964-65; president, Suffolk County Hearing and Speech Center, 1966-71; director of Marvin Josephson Associates, Inc, New York., 1969-77; director of Bank of Babylon, New York, 1973-79; chair, board of trustees, College of New Rochelle, New York, 1974-80; director of Anchor Savings Bank, 1976-91; chair, Council of Governing Boards, 1979-80; commentator, CBS-Radio, 1980-82; television commentator, 1981-82. Member: Board of Education, West Islip, New York, 1953-58; board of directors, Good Samaritan Hospital, West Islip, New York, 1969-78. Honorary Degrees: D. of Pedagogy, Rhode Island College, 1969; D.H.L. Alfred University, 1969; D.F.A., Fordham University, 1975; Litt.D., Indiana State University, 1978; L.L.D., Elmira (New York) University, 1980; D.L., Marquette University, 1983; D.P.S., Central Michigan University, 1984; D.H.L., St. Joseph College, 1987. Honorary Fellow: American Academy of Pediatrics. Recipient: Sylvania Award, 1956; Peabody Award, 1958, 1972, 1979; American Education Award, Education Industries Association, 1978; Distinguished Achievement Award, Georgia Radio and TV Institute-Pi Gamma Kappa, 1978; Emmy Awards,1978, 1981, 1982, 1983, 1984; TV Father of the Year, 1980; James E. Allen Memorial Award, 1981; Distinguished Service to Children Award, 1981; National Education Award, 1982; American Heart Association National Public Affairs Recognition Award, 1987; Frances Holleman Breathitt Award for Excellence, Kennedy Center for the Performing Arts, 1987; Clown Hall of Fame, 1990; AMA Distinguished Service Award, 1991.