James MacArthur was born in Los Angeles, California and as an infant was adopted by playwright Charles MacArthur and his wife, actress Helen Hayes. James himself actually turned out to be Charles MacArthur's biological son, being the product of his affair with another woman. He grew up in Nyack, New York along with Charles and Helen's daughter Mary (born 1930). Helen Hayes herself later guest starred in a 1975 episode of Hawaii Five-O, playing Danny's aunt. Mary was an inspiring stage actress, but died in 1949 from polio at the age of 19.
MacArthur was a graduate of Allen-Stevenson School in New York and Solebury School in Pennsylvania.
MacArthur made his stage debut in Olney, Maryland in 1949.
In 1968, when Leonard Freeman made a pilot episode of Hawaii Five-O, and presented it to test screening. The reaction to Five-O was positive, except for actor Tim O'Kelly, who played Danny in the pilot episode. Freeman remembered MacArthur's portrayal of a travelling preacher in Hang 'Em High, thus he called MacArthur and offered him the role of Danny.
MacArthur played Danny in seasons 1-11, before returning to theatre in 1979. Hawaii Five-O itself ended after the next season in 1980.
In 1997, MacArthur was contacted by Stephen J. Cannell, who was back then making a new Hawaii Five-O pilot episode with Gary Busey and Russell Wong starring. In this episode, MacArthur would again play Danny, who had become the Governor of Hawaii in the plot. However, the episode did not air.
James MacArthur died on October 28, 2010 of natural causes in Jacksonville, Florida. Thus the next episode of the reimagined 2010 Hawaii Five-0, "Hoapono", which aired on November 1, was dedicated to him - "In Memorium James MacArthur 1937-2010 The Original 'Danno'" appeared on screen before the episode started.