From my friends at Hollywoodmemoir.com
James Gordon MacArthur (December 8, 1937 - October 28, 2010) was an American actor known for the role of Danny "Danno" Williams, the reliable second-in-command of the fictional Hawaiian State Police squad Hawaii Five-O.
Hawaii Five-O ran for twelve years—eleven with MacArthur.
http://www.hollywoodmemoir.com/james-macarthur