Rhys Williams made his debut for Cardiff RFC in 1999 and stayed within the set-up after the inception of regionalism and the creation of the Cardiff Blues region in 2003. He has been an ever-present figure in the Cardiff and subsequent Blues line up since his debut, and was Captain in the 2005–2006 season.

Williams has forty-four Wales caps, scoring a try in his most recent test against Canada in 2005. He lost his national squad place in the 2005–2006 season and was out of contention for the 2006 Invesco Perpetual Series after tearing the muscle off his thigh bone in a Blues' comeback game from a hip injury in October 2006.

Williams returned from this injury late in the 2006–2007 season; he made a superb man-of-the-match comeback in the Magners League clash with Edinburgh in April 2007, scoring a first-half hat-trick in the 48-0 victory.

Williams was part of the Commonwealth Games Sevens squad that played in Melbourne in March 2006, and defeated South Africa 29-28 to secure the Plate title.