He played as a prop for Wakefield Trinity Wildcats, St Helens, Castleford Tigers (two spells) and Huddersfield Giants, as well as Brisbane Norths, and for Melbourne Storm in the National Rugby League (NRL).

Mason played in three Challenge Cup finals, winning one in 2004 for St Helens against Wigan in Cardiff.

In 2009 Mason won the 'Coaches' Player of The Year', the 'Directors' Player of The Year', and the Huddersfield Giants 'Man of Steel Award',[citation needed] he also appeared in a Challenge Cup final in the same year at the Wembley Stadium against Warrington Wolves, a boyhood dream come true for Mason, also appearing in 2 more Challenge Cup finals in 2004 which he won for St Helens against Wigan at the Millennium Stadium playing prop.

Keith Mason won caps for Wales while at Wakefield Trinity Wildcats in 2001 against England, and while at Melbourne Storm in 2002 against New Zealand in 2002.

[4] At the end of the 2013 season, Mason retired from playing rugby league in order to pursue a career as an actor.