Zainuddin played as a striker for the Bruneian representative team in the Malaysia league in the 80s and 90s.

[2] He made several appearances for the national side, and scored a goal in a 1986 World Cup qualifier against Hong Kong.

After a coaching stint for Brunei's M-League side while still registered as a player in 1988,[3] he took charge of the national team for 6 games at the 2002 World Cup qualifying.

[4] He then turned to club football in 2008 with AM Gunners,[5] coaching them until 2011 when the season was aborted abruptly.

[7] In 2020, he was appointed assistant coach of the Brunei national football team headed by Ali Hj Mustafa.