MacArthur grew up around the literary and theatrical talent. Lillian Gish was his godmother, and his family's guests included Ben Hecht, Harpo Marx, Robert Benchley, Beatrice Lillie, John Barrymore and John Steinbeck. 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.
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.
At the age of 18, he played Hal Ditmar in the television play, Deal a Blow, directed by John Frankenheimer and starring Macdonald Carey, Phyllis Thaxter, and Edward Arnold. In 1956, Frankenheimer directed the movie version of the play, which was renamed The Young Stranger, with MacArthur again in the starring role. Again, his performance was critically acclaimed, earning him a nomination for Most Promising Newcomer at the 1958 BAFTA awards.
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.
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. For a period in the 1990s, he was part owner of Senior World publication, as well as writing the occasional Celebrity interview. He continued to appear at conventions, collectors' shows, and Celebrity sporting events. A keen Golfer, he was the winner of the 2002 Frank Sinatra Invitational Charity Golf Tournament.
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 remained active in theatre, 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. He also released several records in the early 1960s, scoring two minor hits with "(The Story of) The In-Between Years" and "The Ten Commandments of Love", both of which peaked at number 94 in the Billboard Hot 100.
In 1966, he guest-starred as Lt. Harley Wilson in "The Outsider", episode 20 in the second season of Twelve O'Clock High (TV series). He co-starred with his mother Helen Hayes in the 1968 episode "The Pride of the Lioness" on the Tarzan television series. MacArthur also gave a particularly chilling performance as baby-faced opium dealer Johnny Lubin in The Untouchables episode, "Death For Sale".
He then went on to roles in movies including The Interns, Spencer's Mountain, The Truth About Spring, and Cry of Battle, as well as The Love-Ins and The Angry Breed. 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. In 1963, he was nominated for the "Top New Male Personality" category of the Golden Laurel Awards 1963
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.
After leaving Hawaii Five-O, McArthur guest-starred on such television shows as Murder, She Wrote, The Love Boat, Fantasy Island, and Vega$, as well as in the miniseries Alcatraz: The Whole Shocking Story and The Night the Bridge Fell Down, and in the television movie Stormchasers: Revenge of the Twister (1998), with Kelly McGillis.
In 2001, a Golden Palm Star on the Palm Springs Walk of Stars was dedicated to him.
The episode "Ho'apono" from the 2010 version of Hawaii Five-0 was dedicated to MacArthur.
