Ainsworth saved Rangers from a fifth consecutive defeat after goals from Billy Paynter and Ryan France put the visitors in command.
The match had been levelled by the time Myhill was adjudged to have handled the ball out of the area 14 minutes from time.
Having looked on course for an easy victory, Hull were left hanging on for a point.
They went ahead just before half-time. Craig Fagan crossed brilliantly and France headed past Simon Royce and into the corner of the net.
Hull had earlier come close to scoring when Leon Cort headed Curtis Woodhouse's corner wide of the target.
Rangers' best moment of the first half was Paul Furlong's rasping drive just over the bar following a well-worked free-kick.
Veteran striker Furlong, back after a four-game ban, also had a header saved by Myhill.
France's goal was a blow for the home side who then lost defender Georges Santos through injury.
Yet Rangers almost scored just before the break, when Marcus Bignot's cross was volleyed wide by Ainsworth.
Rangers were dealt another blow when skipper Kevin Gallen limped off just after the break and in the 50th minute Hull doubled their lead. Again QPR's undoing was a fine hanging cross. This time from the right and the boot of Mark Lynch, whose delivery was headed home by Paynter.
Ian Holloway's side quickly pulled one back. Richard Langley's corner found Ainsworth who headed in his third of the season.
But Rangers were caught napping soon after and almost conceded a third. France breezed past Bignot and chipped Royce, but Martin Rowlands was on hand to clear to safety.
Having survived that scare the R's struck lucky with a leveller after 65 minutes. Ainsworth's shot a wicked deflection that left Myhill stranded.
Rangers were unlucky not to go ahead when Lee Cook's 30-yard drive struck the post, but luck was back on their side when Myhill was dismissed.
It seemed a harsh decision and worse almost followed for Hull from the resulting free-kick, which Langley smashed past substitute keeper Matt Duke and on to the post.
Rangers piled forward but were unable to find a winner. Langley fired just wide and Duke kept out Furlong's acrobatic effort in injury time.