He returned to Honolulu in 1965 and became law clerk to United States District Court Judge Martin Pence.
He left public service to become a partner in the law firm Libkuman, Ventura, Moon and Ayabe where he stayed until 1982.
It was from the law firm that Governor George Ariyoshi appointed Moon to the Hawaii State Judiciary as a circuit court judge.
Governor John Waihee then elevated Moon to the office of Associate Justice of the Hawaii State Supreme Court in 1990.
His decision did not gain a majority, but Court of Appeals Judge James S. Burns, who was filling in for a recused Justice, wrote a concurrence agreeing with the result of Moon's opinion.