He attended Grosvenor Grammar School, not traditionally a football-playing school, but despite this, made a name for himself at Crusaders.
He later revealed that at 15, he had the choice of signing for either or Glentoran. He made his first-team debut against Chimney Corner in the County Antrim Shield on 29 October 2002, and scored on his league debut, a 2-4 defeat to Glentoran on 9 November. He also scored a late equalizer in normal time in the County Antrim Shield final against Linfield.
Coates was instrumental in " triumph in the 2012 Setanta Sports Cup.
In the semi-final second leg against Sligo Rovers, he scored an extra-time penalty which put into the final. In the final itself against Derry City, he scored twice in a 2-2 draw, and scored in the resulting victorious penalty shoot-out in a man of the match performance.
He also scored in the 2012 Irish Cup Final in a 1-4 defeat to Linfield. In May 2010, Coates was called up to the Northern Ireland squad for friendly matches against Turkey and Chile (against both of whom he played as a second-half substitute).
He also appeared in a friendly against Morocco, and signed a contract that same season with to keep him at the club until 2014.
As of 11 August 2011, he has received 6 caps, and is the most capped player of all time. Club Individual.