His first radio role was on the Theatre Guild on the Air, in 1948. Theatre Guild on the Air was the premier radio program of its day, producing one-hour plays that were performed in front of a live audience of 800. Hayes accepted a role in one of the plays, which also had a small role for a child. Her son was asked if he would like to do it, and he agreed.

Born in Los Angeles, MacArthur was the adopted son of playwright Charles MacArthur, and his wife, actress Helen Hayes. He grew up in Nyack, New York, along with the MacArthurs’ biological daughter, Mary; she died of polio in 1949. He was educated at Allen-Stevenson School in New York, and later at the Solebury School in New Hope, Pennsylvania, where he starred in basketball, football and baseball.

MacArthur made his stage debut at Olney, Maryland in 1949, with a two-week stint in The Corn Is Green. His sister Mary was in the play and telephoned their mother to request that James go to Olney to be in it with her. The following summer, he repeated the role at Dennis, Massachusetts, and his theatrical career was underway.

In 1954, he played John Day in Life With Father with Howard Lindsay and Dorothy Stickney. He became involved in important Broadway productions only after receiving his training in summer stock theatre.

In 1955, at the age of 18, he played Hal Ditmar in the television play, ‘”Deal a Blow”, an episode of the series Climax! directed by John Frankenheimer and starring Macdonald Carey, Phyllis Thaxter, and Edward Arnold. Response was excellent, the New York Times saying he “performed splendidly”.

In late 1956 it was announced MacArthur would make Underdog, based on a novel by W. R. Burnett, alongside his mother and Susan Strasberg but it was not made.

MacArthur was selected by Walt Disney to star in The Light in the Forest (1958), playing a white man raised by Native Americans. In April 1957 MacArthur signed a three-picture deal with Disney as part of his casting. On Light in the Forest he was paid $2,500 a week. This went up to $3,000 a week for the second film and $3,500 for the third, although he could not be forced to work other than during his summer vacation at Harvard, where he was studying history.

In his final year at Solebury, MacArthur played guard on the football team; captained the basketball team; was president of his class, the student government, and the Drama Club; rewrote the school’s constitution; edited the school paper, The Scribe; and played Scrooge in a local presentation of A Christmas Carol. He also started dating a fellow student, future actress Joyce Bulifant; they were married in November 1958 and divorced nine years later.

MacArthur made his Broadway debut in 1960, playing opposite Jane Fonda in Invitation to a March, for which he received a Theatre World Award. Although he never returned to Broadway, he would remain active in theatre throughout his career, appearing in such productions as Under the Yum Yum Tree, The Moon Is Blue, John Loves Mary (with his then wife, Joyce Bulifant), Barefoot in the Park, and Murder at the Howard Johnson’s.

In 1963, he was nominated for the “Top New Male Personality” category of the Golden Laurel Awards 1963. That year he starred in and produced a pilot for a TV series about a writer, Postmark: Jim Fletcher (1963), but it was not picked up.

MacArthur was in Ride Beyond Vengeance (1966) and guest starred in Branded, Combat!, Gunsmoke, Hondo, Insight, Death Valley Days, Bonanza and The Virginian. In 1966, he guest-starred as Lt. Harley Wilson in “The Outsider”, episode 20 in the second season of Twelve O’Clock High. He co-starred with his mother Helen Hayes in the 1968 episode “The Pride of the Lioness” on the Tarzan television series.

MacArthur’s first wife was Joyce Bulifant. On the set of The Angry Breed, in 1968, MacArthur met Melody Patterson, who was to become his second wife. They were married on the Hawaiian Island of Kauai in July 1970, and divorced five years later. His third wife was former LPGA golfer Helen Beth Duntz.

Throughout his career, MacArthur had also found time for various other ventures. From 1959 to 1960, he partnered with actors James Franciscus and Alan Ladd, Jr. in a Beverly Hills telephone answering service. In June 1972, he directed the Honolulu Community Theatre in a production of his father’s play The Front Page.

Bored with Hawaii Five-O, MacArthur left the show in 1979, one year before it ended. “It was just time,” he said . “I called the producer from South America and told him I was heading down the Amazon River….”

He appeared in A Bedfull of Foreigners in Chicago in 1984, and in Michigan in 1985. He followed this with The Hasty Heart, before taking a year out of show business.

In 1987, he returned to the stage in The Foreigner, then played Mortimer in the national tour of Arsenic and Old Lace with Jean Stapleton, Marion Ross, and Larry Storch. In 1989, he followed another stint in The Foreigner with Love Letters, and in 1990–1991, A Bedfull of Foreigners, this time in Las Vegas.

He also appeared in television and radio specials and interview programs. His latest appearances included spots on Entertainment Tonight, Christopher’s Closeup, and the BBC Radio 5 Live obituary program Brief Lives, in which he paid tribute to his Hawaii Five-O castmate, the late Kam Fong. In 1997, MacArthur returned without Jack Lord (who was in declining health) to reprise his character, who had become Hawaii’s governor in the plot, in the 1997 unaired pilot of Hawaii Five-O which starred actor Gary Busey.

In 2001, a Golden Palm Star on the Palm Springs Walk of Stars was dedicated to him.

In April 2003, he traveled to Honolulu’s historic Hawaii Theatre for a cameo role in Joe Moore’s play Dirty Laundry. Negotiations were underway in summer 2010 for MacArthur to make a cameo appearance in the new CBS primetime remake of Hawaii Five-O at the time of his death, a role that eventually was given to Al Harrington. On the November 1, 2010, episode, MacArthur’s death was mentioned in a short tribute that played before the start of that episode.

MacArthur died on October 28, 2010, at the age of 72, of cancer, at the Mayo Clinic in Jacksonville, Florida.