After following his brother Clarence "Spot" Horder to South Sydney, Harold in his first game, stepped and swerved through the entire Glebe team in a 90-metre dash to score one of the greatest individual tries in rugby league history. He went on to be the NSW Rugby Football League's top try-scorer in 1913, 1914 and 1917 and for each of the four seasons 1913, 1914, 1918 and 1922 he was the League's top point scorer. Horder was selected to go on the 1921–22 Kangaroo tour of Great Britain. He also scored a double in Norths' 1922 grand final win. He scored 102 tries for South Sydney and 50 tries for North Sydney. In his final season at Souths he became the first player to score 150 tries in NSWRFL history.
The Gregory's reference records that in the 1912 City Cup-tie against Glebe, Harold induced the Souths selectors to name his relatively inexperienced brother Clarrie in the side, While Glebe was concentrating on Harold, Clarrie cut holes in the defence and Souths won 30-5. 
He moved to Brisbane rugby league club Coorparoo as their captain coach for two seasons after leaving Souths.
In 2004 Horder was named by Souths in their South Sydney Dream Team, consisting of 17 players and a coach representing the club from 1908 through to 2004. In August, 2006 he was also named at winger in the North Sydney Bears' Team of the Century.
In February 2008, Horder was named in the list of Australia's 100 Greatest Players (1908–2007) which was commissioned by the NRL and ARL to celebrate the code's centenary year in Australia.