The Yorkshireman is Chris Wilder's third signing of the transfer window.
"Mason's desire to come here and join our fight is huge for me," said Wilder. "He's got a point to prove, he's a local lad who knows what Sheffield United are all about, he has his finger on the pulse and knows what to expect here.
"He's got valuable Premier League experience and can play all across the back which gives us options. Like Ben (Brereton Diaz) and Ivo (Grbic), we expect Mason to play a big part in the remainder of the season."