The diminutive midfielder struck in the 47th minute having been handed a start after the influential Adel Taarabt was sidelined with an illness.
It was no more than the visitors deserved, having dominated most of the game against a Doncaster side who have not won at home since February 1.
And with Norwich failing to win away at Hull, the London club are looking in prime position to secure a return to the Premier League.
Heidar Helguson, who scored both goals in QPR's 2-1 win over Crystal Palace, hit the bar with an early header from Tommy Smith's cross.
The hosts quickly launched an attack of their own and Simon Gillet tested Paddy Kenny with a right-footed effort from just inside the area.
And soon after it was their turn to be denied by the woodwork as Franck Moussa hit the upright with a close-range shot following good work from James Coppinger.
The lively tempo continued as Helguson set up Smith for a shot which Gary Woods did well to tip round the post in the 19th minute.
Fitz Hall bravely put his body in front of a powerful Coppinger shot and Alejandro Faurlin saw his ambitious 25-yard drive keep rising up and over the bar.
The visitors should have taken the lead on the half-hour mark but Ephraim contrived to turn Wayne Routledge's pinpoint cross wide of the near post from just six yards out.
And Kenny was called upon to make one final save in the first half when he tipped over a powerful header from Shelton Martis.
It took just two minutes of the second half for QPR to take the lead as Routledge and Faurlin linked well down the right before finding Ephraim, who stepped inside his man and fired in a low shot from just inside the area.
Coppinger continued to threaten with his direct running and Clint Hill was forced into a timely intervention on the hour mark to prevent the tricky winger from creating a shooting opportunity as Rovers sought a way back into the match.
Ephraim was withdrawn shortly after, with Akos Buzsaky sent on to rejuvenate Neil Warnock's side.
Despite controlling large parts of the game, Rangers' one-goal advantage still looked vulnerable and Kenny had to react quickly to claim substitute Ryan Mason's cross.
Sean O'Driscoll attempted to rally his charges with the introduction of veteran striker Jason Euell, whose presence ruffled a few feathers in the Rangers defence as he saw a late header fly just wide.
But QPR held on to complete the double over Doncaster, who may yet find themselves embroiled in a relegation battle.