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. Despite being from Newcastle, Mason was unable to join the Newcastle Knights because the club had a policy against signing players who had been affiliated with the Hunter Mariners during the Super League war. This policy forced Mason, at 17 or 18 years old and having recently lost his father, to leave his hometown and family to pursue his rugby league career in Sydney, though he would have preferred to play his entire career for Newcastle. After the
2002 NRL season, Mason was selected to make his debut for
Australia against
Great Britain. He scored a try on debut. During 2003, Mason earned a call-up to make his debut for
New South Wales in Game 3 of the
2003 State of Origin series. After the
2003 NRL season, he went on the
2003 Kangaroo tour of Great Britain and France, helping Australia to victory over
Great Britain in what would be the last time the two nations contested an
Ashes series. In 2003, he tested positive to an illicit social drug, reported to be an
amphetamine, and was fined $25,000 by his club. Mason has never commented on the issue publicly. In 2004, Mason along with
Trent Waterhouse,
Craig Wing,
Craig Gower and
Mark O'Meley, were fined for unprofessional behaviour and for leaving the team hotel after the side had returned from the night out. Mason forged another player's signature at an autograph event, and after a heavy drinking session with teammates, reportedly visited a
brothel. The next season Canterbury-Bankstown reached the
2004 NRL Grand Final and triumphed 16–13 over the
Sydney Roosters. Mason was awarded the
Clive Churchill Medal for
Man of the Match. Reflecting on his Clive Churchill Medal win in the 2004 NRL Grand Final against the Sydney Roosters, Mason stated it was the "first time I actually felt bad of winning something" due to many opposing players being his long-time friends and former junior teammates from Newcastle, including Anthony Minichiello, Brett Finch, and Luke Ricketson. He played in the
2005 Tri-nations tournament. In June 2006, news speculation surfaced that Mason was considering joining the United States'
National Football League (NFL), a childhood dream. He was said to be courted by several teams, including the
Oakland Raiders and
San Francisco 49ers. In July it was confirmed the
New York Giants had offered him the opportunity to trial with them after the Rugby League Tri-Nations series finished in November. However, the Canterbury club objected to the deal, and Mason remained with the club beyond 2006. In 2006, Willie returned to become a regular member of the Bulldogs team where he managed to play 20 club matches, all three Origin games for NSW – in which he was named
Man of the Match in the opening game and three Tests for Australia in the
2006 Tri-Nations series. He was a firm figure in the clubs return to finals football as the Bulldogs finished second on the ladder, Canterbury-Bankstown were beaten by eventual Premiers the
Brisbane Broncos one game shy of the Grand Final. Mason was televised appearing to swear during the New Zealand
haka, before game 1 of the
2006 Gillette Tri-nations Series. Later in the match Mason suffered a swollen black eye and suspected broken eye socket from a shoulder charge by
David Kidwell. Mason explained that his swearing was in response to the traditional
Māori dance being performed by
Brent Webb, an Indigenous Australian of
Torres Strait Islander origin who became a New Zealand citizen. Mason was banned for one game and fined $5,000 following an altercation with
Stuart Fielden during Great Britain's victory over Australia in November 2006. In December 2006 Mason took part in a trial with the
New York Jets, but still maintained he would fulfil his contract with the Canterbury club until the end of the 2009 season. Mason said "I'm probably at the time in my career now where maybe it is time for a change. New York City is the capital of the world. I'm pretty sure I could settle in there if given the opportunity..." However a Jets representative stated that Mason even being offered a contract was very unlikely, "I don't think they were even football drills, it was more of a visit. We just had a look at him and that is where we left it." The Jets coach
Eric Mangini was impressed with Mason, saying "Willie is a guy that came to our attention, he is a pretty good rugby player, and it was just one of those opportunities we had, where we have to take a look at him. What we're doing is taking a look at him, seeing what potential there is for him. He seems to have been a pretty dynamic guy in that league for a long time. He seemed relatively feisty when I spoke to him." The Bulldogs never anticipated any move, saying "Willie has given us his guarantee he will be with us." Mason was selected to play for the
Australian national team at second-row forward the 2007
ANZAC Test match victory against New Zealand. A few weeks later he was involved in a nightclub altercation with the out-spoken
Richie Williams, who Mason claimed shows no respect to the older players in the NRL and is a cocky kid who has yet to prove himself in the NRL. Mason was voted the most hated player by a fans poll in 2007. At
Sydney Airport in 2007, Mason verbally challenged a female reporter from radio station
2GB, resulting in a complaint from 2GB to the
Canterbury-Bankstown club and the
ARL Mason later admitted to the outburst, which was captured on audio recordings, and apologised to 2GB journalist Michelle Keighran. Mason is sponsored by
sportswear manufacturer,
Champion, which almost severed its contract with the controversial footballer as a result of the incident. In 2007, Mason was selected for all three Origin games for NSW and represented Australia in the
ANZAC Test match. At the conclusion of the
2007 NRL season, the media began reporting that Mason was unhappy at the Canterbury club. On 31 October 2007
The Daily Telegraph reported that Mason wanted to leave the Bulldogs after an altercation with the club's CEO, Malcolm Noad. Speculation that Mason was to be released from his contract led to a number of English and Australian clubs confirming they were seeking to sign Mason to a contract for the 2008 season. After the Roosters were able to release back-rower
Ashley Harrison to the Gold Coast Titans and considered releasing other players, the club was able to make an offer for Mason. On 15 November 2007 it was confirmed that Mason had signed a three-year deal with the
Sydney Roosters.
Sydney Roosters In February 2008, Mason and his new Roosters teammate
Anthony Tupou were charged with urinating in a public place during a visit to
Port Macquarie. 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. Mason missed the Roosters first game of the 2009 season after they were thrashed by
South Sydney Rabbitohs 52–12. Mason returned in Round 2, against the
Canberra Raiders they were victors 30–4. In May 2009 Mason was fined $2,000 by
Sydney Roosters after being photographed urinating in a public place. He was also dropped to the
Newtown Jets. The Roosters finished the year in last spot. 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. He made his Cowboys debut off the bench in a 30–24 loss to the Broncos in Round 1, 2010.
Super League In September 2011, Mason signed a three-year contract with
Hull Kingston Rovers, pending a successful working visa application. Six months into his contract, Hull KR were forced to release Mason as he was unable to obtain a Tongan passport that would have enabled him to come off their overseas quota.
Newcastle Knights 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 victory over 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 two years. On 10 March 2014, Mason was disqualified from driving for six months and fined $900 for drink driving. The offence was in relation to Australia Day of that year where after a marathon drinking session, Mason drove to training, was pulled over by police and blew a blood alcohol reading of 0.09.
Manly Warringah Sea Eagles On 28 October 2014, after a three-year stint with the Knights, Mason signed a one-year contract with the
Manly Warringah Sea Eagles starting in 2015. On 13 May 2015, Mason was in the spotlight again, this time for being grabbed by Newcastle player
Korbin Sims on the genitals in Manly's 30–10 loss to Newcastle at
Brookvale Oval. Sims tried to laugh off the incident by saying "I apologise to the fans, the NRL and people involved with the game and the club, if anyone found it inappropriate", Sims told Fairfax Media. "It was merely meant to be a harmless gesture between two friends." In 2018, Mason spoke to the media about his time at Manly in relation to the ongoing saga with the board infighting and coach Trent Barrett claiming the facilities at the club were not up to date. Mason said "I spent my last year there in 2015 so I understand where a lot of the backlash is coming from, back then we were getting changed out of our cars, out of our boots. It was pretty unprofessional. Only three years ago you're getting changed out of your f****n boot. It's ridiculous and they've only got their shit together now because Trent Barrett kicked up a stink. We had to share weights with the Australian touch team and union, there was about four or five clubs in there. I'd hate to be a part of the club at the moment... it's a very proud club but it's in shambles at the moment. I feel sorry for all the players".
Catalans Dragons On 16 February 2016, Mason returned to the Super League with
Catalans Dragons, signing a one-year contract starting effective immediately. ==Rugby union==