The 22-year-old will move to the Reebok Stadium until January 5 and could make his debut against Middlesbrough on Saturday.
Mason almost made his a loan move to Bolton earlier this month but the deal was called off by Cardiff manager Malky Mackay because of injuries to Rudy Gestede and Peter Odemwingie.
However, the former Plymouth Argyle man has now completed his switch and joins a side who are eight games unbeaten in the Championship.
The most recent victory for Dougie Freedman's side came on Saturday with a 1-0 win over Watford, Jermaine Beckford the scorer.
Mason, who moved to the Welsh club in 2011, has not featured in the Premier League this season, instead being limited to two League Cup appearances.
Prior to Cardiff's promotion, Mason made 69 Championship appearances for the club in two seasons, scoring 14 league goals.
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