The ex-Bury hit-man has been a long term target for Neil Warnock since the pair spent time together whilst at Gigg Lane. The Blades boss turned his attention to Johnson after Burnley won the race to sign Stoke's Ade Akinbiyi.
Johnson, however has struggled this season, netting just six goals in 37 appearances and which has prompted him to be linked with Wolverhampton Wanderers and Hull City has he looks for a move away from the City Ground.
Meanwhile Neil Warnock has revealed that Johnson's agent rang the United manager to tell him that the striker was available to him for which Neil took no time in snapping him up.
"I had to make a quick decision on whether I wanted to sign him and I couldn't see anything against it - we cannot lose on the deal," Warnock told sufc.co.uk.
"He has not been guaranteed a place in the first team, but he wants to take his chances here.
"He gives us an extra body in the squad and all the lads were pleased to see him when he arrived for training."