Born in Auckland, New Zealand to a mother of Tongan, Samoan and African American descent and an Australian father, Mason moved to Australia at a young age and grew up in Toronto, New South Wales playing his junior football for the Toronto Macquarie Scorpions and the Lakes United Seagulls.
In 1997 Mason was signed by the Hunter Mariners and played in the lower grades until the club became defunct at the end of the year.
In 1998, Mason signed with the Bulldogs and played in the under-18s Jersey Flegg competition. In 1999, Mason suffered a leg fracture in the game before the Bulldogs’ Jersey Flegg Grand Final win. In 2000, Mason moved on to the NSWRL Premier League.
In Round 13 of the 2000 NRL season he made his NRL debut for the Bulldogs against the Penrith Panthers after first-grader Darren Britt suffered an injury. He became a regular first-grader for the rest of the season. After the 2000 season, Mason made his international debut for Tonga, his mother’s country of origin, in the 2000 Rugby League World Cup played in Great Britain and Ireland.
The Roosters dominated through the regular season finishing in 4th spot and in August 2008 Mason was named in both the preliminary 46-man Kangaroos squad for the 2008 Rugby League World Cup and the Tonga training squad. However, in the final regular game of the season Mason ruptured his anterior cruciate ligament in his knee and took no part in the Roosters Finals Campaign.
When Brian Smith took over the coaching role at the Roosters he told Mason he was not in his future plans at the Roosters and his services were no long needed there. Interest in Mason was said to be high with a number of high-profile rugby union and rugby league clubs chasing Mason’s signature, Japanese Rugby Union teams, French Super League team Catalans Dragons and the Queensland Reds were just some of the clubs after Mason. He was given extended leave from the Roosters to find a new club. There were also some rumours he was going to join the Newcastle Knights late in the year, but the Knights denied any relation to him. The North Queensland Cowboys made a late play for Willie Mason and held talks with him on 2 February 2010 If the Cowboys had rejected Mason the Newcastle Knights were poised to make a deal with him as they had lost prop Danny Wicks
North Queensland Cowboys
On 5 February 2010, Mason agreed to a one-year contract with the North Queensland Cowboys. On the same day that Lote Tuqiri announced that he would be returning to the NRL it was announced that Willie Mason had signed a one-year deal with the North Queensland Cowboys. He made his Cowboys debut off the bench in a thrilling 30–24 loss to the Broncos in Round 1, 2010.
On 18 April 2012, it was confirmed Mason had signed with the Wayne Bennett coached Newcastle Knights for the remainder of the 2012 NRL season. He made his Newcastle debut at Hunter Stadium in their Round 8 defeat of the Penrith Panthers. Mason’s younger brother Rodney was also playing rugby league with the Newcastle Knights at the time. On 15 November 2012, Mason re-signed with the Knights for 2 years.
Manly Warringah Sea Eagles
On 28 October 2014, after a three-year stint with the Knights, Mason signed a 1-year contract with the Manly Warringah Sea Eagles starting in 2015.
On 16 February 2016, Mason returned to the Super League with Catalans Dragons, signing a 1-year contract starting effective immediately.
Mason retired at the end of the 2016 season