Matt Smith put the hosts ahead after five minutes and Luke Freeman restored the lead following Joe Newell's stunning equaliser.
Yeni Ngbakoto's penalty four minutes into the second half made it 3-1, before Luongo headed home another eight minutes later and skipper Nedum Onuoha netted in injury time to round off the scoring.
It meant a fourth consecutive home win for Rangers, whose improvement under Holloway continues.
Rotherham are on the verge of relegation and their defensive shortcomings were evident when Smith gave Holloway's side an early lead.
Idrissa Sylla played the ball forward and Smith was afforded time and space to collect the ball near the left-hand edge of the penalty area before applying an emphatic finish, shooting beyond keeper Lewis Price and into the far corner of the net.
Rotherham, whose defeat was their seventh in a row, equalised with a brilliant goal five minutes after Smith's opener.
Danny Ward headed a long ball back towards Newell, who hit the net with a first-time left-footed volley from 30 yards out.
However, more terrible defending by Rotherham led to QPR going back in front within two minutes.
The Millers back-line failed to react when Freeman nudged the ball on after Smith had gone up for a header, and the midfielder himself collected the loose ball and steered it past Price from just inside the penalty area.
Worse followed for the visitors soon after the restart when Aimen Belaid was adjudged to have fouled Smith while attempting to prevent the target man reaching Darnell Furlong's long throw-in.
Ngbakoto calmly steered the resulting spot-kick past Price and then crossed for an unmarked Luongo to nod in the fourth, finally ending the Australian midfielder's goal drought on his 63rd appearance for the R's.
The Championship's bottom side looked completely devoid of confidence and were fortunate not to concede a again before Onuoha eventually completed the rout.
Sylla was on hand to hook the ball towards goal after a fumble by Price, but Belaid cleared off the line.
Substitute Conor Washington then went close when he tricked his way past a couple of challenges and fired against the inside of the post.
The loose ball fell to Ravel Morrison, also on as a substitute, who blasted narrowly wide.
However, the fifth goal soon followed when Onuoha, completely unchallenged, touched in Ngbakoto's left-wing free-kick from close range.