James Gordon MacArthur (December 8, 1937 – October 28, 2010) was an American actor best known for the role of Danny "Danno" Williams, the reliable second-in-command of the fictional Hawaiian State Police squad Hawaii Five-O.
Born in Los Angeles, California, he was adopted as an infant by playwright Charles MacArthur and actress Helen Hayes. He grew up in Nyack, New York, along with the MacArthurs' biological daughter, Mary. 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.
In his final year at Solebury, he 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, Joyce Bulifant; they were married in November 1958 and divorced nine years later.