He was 72 and his family was at his side at the time of his death, according to his agent, Richard Lewis. The actor died in Florida, from natural causes.
Born in Los Angeles on Dec. 8, 1937, MacArthur was adopted shortly after his birth by two living legends: actress Helen Hayes, considered the First Lady of the American stage, and her husband, Charles MacArthur, who, with collaborator Ben Hecht, wrote what some consider the greatest American stage comedies of all time, The Front Page and Twentieth Century.
The young MacArthur grew up with his parents' biological daughter, Mary, on the family estate in Nyack, New York, which was called "Pretty Penny" – so named because that's what it had cost.
Growing up in such an atmosphere was not always easy. As MacArthur told McCall's magazine in 1956 (and reprinted on his official Web site): "I think my mother is a great actress and the sweetest, gentlest and most generous woman I ever met. But she's vague at times and often impractical and sometimes does strange things … She knows practically nothing about handling money."
Though he played sports as a boy, it seemed almost pre-ordained that MacArthur would become an actor. During summer breaks as a student at Harvard in the late 1950s and early '60s he played the clean-cut male love interests in Third Man on the Mountain, Kidnapped and Swiss Family Robinson – all for Walt Disney – and, in 1961, debuted on Broadway opposite another newcomer, Jane Fonda, in the comedy Invitation to a March, which ran 113 performances and won him the Theatre World Award for Best New Actor.
Other Broadway roles followed, as did movies, including the 1965 all-star WWII drama, The Battle of the Bulge and 1968's Clint Eastwood spaghetti Western, Hang 'Em High, in which MacArthur played a traveling preacher.
That was the role that led to his being cast as Detective Dan Williams on Hawaii Five-0, which costarred Jack Lord and ran on CBS from 1968 to 1980. (A new version, starring Alex O'Loughlin and Scott Caan, debuted this fall on CBS and is also a ratings hit.)
Off camera, according to a family statement, "James was an avid tennis player and enjoyed skiing, fishing and hiking. He was a skilled flamenco guitarist and a consummate reader. His passion for playing golf led him to meet and fall in love with his wife, LPGA tour player and teacher, 'H.B.' Duntz."
His wife of more than 25 years, Helen Beth Duntz, survives MacArthur, as do four children and seven grandchildren. (MacArthur's first two marriages, from 1958 to 1967 to actress Joyce Bulifant, and another, from 1970 to 1975 to actress Melody Patterson, both ended in divorce.)
In lieu of flowers, the family requests contributions be made to the Helen Hayes Awards in Washington, D.C.; the Helen Hayes Hospital in Nyack; the Solebury School MOM Fund in New Hope, Penn.; the Palm Desert Community Presbyterian Church, Palm Desert, Calif.; and the Hawaii Theatre in Honolulu.