The 35-year-old, who played six Super League games for Hull KR in 2011 before moving to French rugby union side Toulon, was at Manly last season.
He played 290 NRL games during his 15-year career spent with five clubs, including Canterbury, where he won the 2004 Premiership.
"Willie knows the job a prop has to do," coach Laurent Frayssinous said.
"He is a physical and aggressive player, both in attack and in defence and we have spoken a lot about the quality of our group and he is really excited to join the club."
Frayssinous' Dragons side have lost their first two games in Super League this season, with defeats by Wigan and Hull FC.
Veteran front-rower Mason scored seven tries and played 24 tests for the Kangaroos, including the famous Great Britain 2006 Tri-Series defeat in Sydney where he struck Stuart Fielden and was involved in a scuffle with tourists captain Jamie Peacock.
He also played 13 games for the Blues in State of Origin, scoring once.

Veteran striker Peter Weatherson bagged a first-half brace, including a penalty, to put the visitors firmly in control at Links Park.
Matthew Flynn and Josh Todd both struck from close range around the hour-mark.
Todd added his second of late on, with Jim Chapman's side rising to fourth, one point clear of Queen's Park, while Montrose drop to ninth after Berwick Rangers and Arbroath both won.

Stevens made his home debut in their 19-13 loss to Ealing, having joined the club after leaving financially-stricken London Welsh last month.
"We're pushing for top four, we want top four," the ex-Nottingham, Worcester and Plymouth Albion centre said.
"We want to do really well in the British and Irish Cup and we want to do as well as we can."
He told BBC Radio Jersey: "This club needs to be pushed forward and I think they definitely are on the right road for doing that."
The island side are currently eighth in the Championship, but are just seven points behind fourth-placed Ealing and point further back from Doncaster in third.
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Highways England wants turn the hard shoulder into an extra lane between junction three by Heathrow Airport and junction 12 at Theale to tackle periods of heavy congestion.
The scheme, costing up to Â£862m, will improve traffic flow and ensure "less stressful driving", supporters say.
Opponents claim it will increase congestion and affect the environment.
The motorways are called "smart" because technology will be installed to monitor traffic, provide information to drivers, and ease congestion by using variable speed limits and utilising the hard shoulder.
"Emergency Refuge Areas" will be put in place every 2.5 km (1.6 miles) so drivers can come off the hard shoulder and await recovery.
The first one was introduced on the M42 near Birmingham in 2006 and there is now also one on the stretch of M4/M5 near Bristol.
Chief scientist Alan Stevens at the Transport Research Laboratory in Crowthorne, which carries out modelling for the smart motorways, said: "They help the traffic go better, drivers get shorter and more predictable journeys and less stressful driving.
"We did a lot of modelling work and put people into driving simulators to see how they would respond, and actually we found no problems at all."
John Booth from Reading Friends of the Earth said smart motorways are not the solution to congestion.
"The reason they're doing this is they want to cater for more traffic on the motorway and will encourage more traffic growth, and that will have a knock on effect on air quality, on climate change emissions, on noise and on congestion on the non-motorway network," he said.
Following written representations for six months and a three-month public reaction period, the decision will be down to Transport Secretary Patrick McLaughlin.
Catalans Dragons have signed former Australia and New South Wales prop Willie Mason on a one-year deal.
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Annan Athletic moved into the League Two play-off places, routing Montrose.
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New Jersey signing Heath Stevens says the club are capable of reaching the Championship play-offs.
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A planning inquiry is under way to create a "smart motorway" along the M4 in Berkshire.