Having suffered cruciate ligament damage in just his third game for the club, the 32-year-old former England international could only watch on helplessly as the west Londoners were relegated last season.
The hosts, including former captain Joey Barton for the first time in 447 days, showed a steely determination rarely seen during their tumultuous two-year stay in the Premier League, refusing to crumble after impressive debutant Atdhe Nuhiu gave Wednesday an early lead.
Nedum Onuoha drew QPR level in the 40th minute when he directed home a Junior Hoilett strike, before Johnson netted his first goal since November 2011 three minutes later.
Substitute Jacques Maghoma rattled the post as Wednesday pushed for a leveller but QPR held firm to pick up their first win since March 9.
Hoilett and Johnson had chances as QPR began brightly, but Nuhiu, who spent time on trial with Premier League new boys Crystal Palace before joining Wednesday, stabbed wide in the 13th minute.
It was a warning shot QPR failed to heed and the 6ft 6in striker opened his account seven minutes later, cutting in from the right before firing an excellent driven effort past Robert Green.
The Hoops wasted a fine chance to level as the half-hour mark approached, with Barton sending an exquisite right-wing cross that Hoilett could only fire over.
The Rs continued to push and, after Michail Antonio's penalty appeals were waved away, managed to level in the 40th minute.
Hoilett, proving a constant threat on the left flank, cut inside and struck goalwards, with defender Onuoha directing home from close range - his first goal since November 2010.
Wednesday returned for the second half strongly and Antonio should have levelled when put through clear on goal shortly after the restart.
Redknapp introduced new signing Charlie Austin for the remaining 20 minutes and the former Burnley striker forced Kirkland into a save shortly after his introduction.
He headed just over as the clock wound down but despite his best efforts, and those of Hoilett and Armand Traore, there were no more goals in Shepherds Bush.