Following a rigorous medical on Monday at Loftus Road, the former Watford man put pen-to-paper on his short-term deal with the West London outfit.
Johnson was released by The Hornets in the autumn before moving on to Division Two club Colchester United for a short spell.
He then moved on to Division One outfit Stoke City before mutually agreeing to be released from his contract with The Potters earlier this month.
After signing his contract with QPR, Johnson revealed his anticipation at helping Ian Holloway's side with their push for promotion this season.
"At this stage of the season, it is not about performances, it is about results," Johnson told the club's official website.
"You have to scrap, battle and fight for everything.
"I've been training so I'm in pretty good shape, I've missed the last three games because I have been suspended due to getting sent off for Stoke against Wimbledon, but I feel okay and I can't wait to get stuck in."
Johnson could be joined at Loftus Road by Burnley defender Arthur Gnohere, who has re-opened talks with the Second Division outfit after initial discussions broke down last week.
The former Caen player's representatives are due at QPR on Wednesday to discuss a possible deal, with Gnohere having spent time on loan with the club earlier this season.
Holloway is keen at add the Frenchman to his squad following a serious knee injury to Danny Shittu and QPR chief executive David Davies is cautiously optimistic that a deal can be struck.
"Arthur is the player we would like to bring to Loftus Road, but obviously any deal has to be right for all concerned," Davies told the club's official website.
"We will sit down with the player's representatives and hopefully talks will reach a satisfactory conclusion."
