Johnson last featured for the Hoops in September as the Loftus Road outfit were relegated to the Championship, but the former Fulham frontman wants to earn an immediate return to the top flight.
The 32-year-old told BBC London 94.9: "It's been a long time for myself. I am a loyal player and a lot of the staff here put a lot of work in to get me back fit.
"I feel like I owe the club and want to help them get back in the Premier League, where they belong."
The former England international made his return during QPR's pre-season fixture at Exeter City.