Born in Pittsburgh, McCargo attended Boston's West Hills College. In 1951, she married Richard Cantrell Moses, who later became an advertising executive in Los Angeles. They had four sons: actors Rick and william R. Moses; Director Harry Moses; and Graham Moses. They were divorced in 1963.

McCargo first entered acting as a supporting player on such popular television shows as Perry Mason (in 1964 she played murder victim Sibyll Pollard in "The Case of the Latent Lover"; and in 1965 she played defendant Louise Selff in "The Case of the Wrathful Wraith".) Her other television show appearances included: Hawaii Five-O, Hogan's Heroes, Voyage to the Bottom of the Sea, Mannix, and The Man from U.N.C.L.E..

McCargo made her feature film debut in the crime comedy Dead Heat on a Merry-Go-Round in 1966, which was also the debut film of Harrison Ford. Subsequent film roles included: Buona Sera, Mrs. Campbell in 1968 (playing opposite Peter Lawford, Gina Lollobrigida, Shelley Winters, Telly Savalas, and Phil Silvers); The Undefeated in 1969 (with John Wayne and Rock Hudson); and Doctors' Wives in 1971. McCargo also became known for her television role as Harriet Roberts on the nighttime soap, Falcon Crest.

In 1970, McCargo married U.S. Congressman Alphonzo E. Bell Jr. of California, a widower with three sons of his own. They had met while she was starring with John Wayne in The Undefeated, Wayne being a close personal friend of Bell's. She then retired from acting to become a political wife.

Using the married name Marian McCargo Bell, she was active politically during his eight-term congressional career and campaigned for her husband, including during his unsuccessful bid for the U.S. Senate in 1976.

McCargo died of pancreatic cancer in 2004, in Santa Monica, California, just eighteen days before her husband Alphonzo.