[citation needed] In his final year at Solebury, MacArthur played guard on the football team, captained the basketball team, rewrote the school's constitution, edited the school paper, played Scrooge in a local presentation of A Christmas Carol and was president of his class, the student government and the drama club.

[citation needed] MacArthur's first radio role was on the Theatre Guild on the Air in 1948, accompanying his mother Helen Hayes.

[6] In late 1956, it was announced that MacArthur would make Underdog, based on a novel by W. R. Burnett, along with his mother and Susan Strasberg, but the project never materialized.

[7] MacArthur returned to television to appear in World in White (1957) and episodes of General Electric Theater, Studio One in Hollywood and Westinghouse Desilu Playhouse.

[9] Deciding to make acting his full-time career, he left Harvard in his second year to appear in two more Disney movies, Kidnapped (1960) and Swiss Family Robinson (1960).

[10] MacArthur made his Broadway debut in 1960 playing opposite Jane Fonda in Invitation to a March, for which he received a Theatre World Award.

[11] He also released several records in the early 1960s,[12] scoring two minor hits with "(The Story of) The In-Between Years" and "The Ten Commandments of Love", which peaked at number 94 on the Billboard Hot 100 in 1963.

[2] MacArthur delivered a chilling performance as baby-faced opium dealer Johnny Lubin in The Untouchables episode "Death For Sale".

He appeared in episodes of The Dick Powell Theatre, Sam Benedict and Arrest and Trial, then made Spencer's Mountain (1963) at Warner Bros. with Henry Fonda and Cry of Battle (1963) in the Philippines.

He appeared in Ride Beyond Vengeance (1966) and guest-starred on Branded, Combat!, Gunsmoke, Hondo, Insight, Death Valley Days, Bonanza, The Virginian, Twelve O'Clock High and Tarzan.

Tim O'Kelly was originally cast as Jack Lord's assistant, but test audiences felt that he was too young, so MacArthur was offered the role.

"[16][17] William Smith, who replaced him on the show, claimed that MacArthur quit "because Jack Lord wouldn't let him have a dressing room.

"[18] After leaving Hawaii Five-O, McArthur guest-starred on television shows such as Time Express, Murder, She Wrote, The Love Boat, Fantasy Island, Walking Tall, The Littlest Hobo,Vega$ and Superboy.

[20] From 1959 to 1960, MacArthur partnered with actors James Franciscus and Alan Ladd, Jr. in the ownership of a Beverly Hills telephone-answering service.

In April 2003, he traveled to Honolulu's historic Hawaii Theatre for a cameo role in Joe Moore's play Dirty Laundry.

Negotiations were under way in Summer 2010 for MacArthur to make a cameo appearance in the new CBS primetime remake of Hawaii Five-0 at the time of his death, a role that eventually was offered to Al Harrington.