The game exploded into life after a slow start when Clark Robertson headed home against his former employers.
And the Blues were in fine form after the break, with another ex-Miller, George Hirst, setting up one goal for Marcus Harness before finding the net himself.
Danny Cowley made five changes from the side that were beaten on the road at Cheltenham at the weekend.
Hirst, Michael Jacobs, Joe Morrell, Louis Thompson and skipper Robertson all returned to the starting line-up.
It meant that Tyler Walker, Aiden O’Brien, Ryan Tunnicliffe, Shaun Williams and Mahlon Romeo dropped to the bench.
Former Pompey players Michael Smith and Joe Mattock were both named in the visitors’ team, who had Richie Barker among their coaching staff.
There were little of note in the opening exchanges besides a flurry of bookings, with both sides ending the first half with two yellow cards each.
In fact, there were 20 minutes on the clock before either side seriously threatened to break the deadlock.
It was the hosts who were first to go close, though, after Jacobs showed a neat piece of skill to take out three defenders.
He then found Ronan Curtis, who cut the ball back for Harness and his effort was kept out at point-blank range by Viktor Johansson.
Rotherham had a few bright moments at the other end of the pitch, without being able to properly test Gavin Bazunu.
And it was the Blues that broke the deadlock on 35 minutes, as ROBERTSON haunted his former club by rising to meet Jacobs’ free-kick and guide a header into the net.
Morrell’s pass then sent Hirst racing away late in the half and the striker did well to hold off a couple of challenges before dragging a shot wide.
The Millers had not shown much attacking flair, although a dipping Chiedozie Ogbene effort just before the break landed on the roof of the net.
Rotherham United 0
Robertson needed to show his capabilities at the other end of the pitch when the action restarted and did well to prevent Smith getting a shot away.
There were then a couple of dangerous attacks for the Blues, but one good move ended with the otherwise impressive Hirst getting his attempt all wrong.
The Millers were trying to find a way back into the contest and Mattock glanced a header wide after meeting Dan Barlaser’s free-kick.
But the Blues soon came close to hitting their opponents on the break and Hirst could not quite get on the end of a Jacobs delivery.
And they did double their advantage just before the hour mark, courtesy of some superb play from Hirst.
The striker raced into space to collect a Bazunu throw before surging forward and holding off two challenges, then sending in a brilliant low ball for HARNESS to slot home.
They might have gone further ahead when a Jacobs corner was only partially cleared, but a thunderbolt from Hayden Carter was just about turned behind by Johansson.
It was only a brief respite for the beleaguered Millers, however, with Pompey soon finding a third goal.
Morrell lifted a pass into the box and Sean Raggett beat Johansson to the ball, nodding it back for HIRST to neatly hook home.
A first substitution soon followed for the hosts, with Friday’s contest perhaps in the mind of Cowley as Jacobs was swapped with O’Brien.
Harness and Thompson also made way to warm applause soon after, as Romeo and Williams were handed opportunities to impress.
The first of those newcomers was soon in the action, although not in the way he intended, with an accidental shove on David Rock sending the official tumbling to the turf.
Pompey were seeing out the closing stages with relative ease and Bazunu made sure of the clean sheet by clawing away a late Barlaser free-kick.
Pompey (4-2-3-1): Bazunu; Carter, Raggett, Robertson (c), Ogilvie; Morrell, Thompson (Williams 76); Harness (Romeo 75), Curtis, Jacobs (O’Brien 69); Hirst
Goals: Robertson 35, Harness 59, Hirst 65
Booked: Robertson, Jacobs
Subs not used: Webber, Mingi, Tunnicliffe, Walker
Rotherham (3-1-4-2): Johansson; Ihiekwe, Wood (c), Mattock; Barlaser; Ogbene, Lindsay, Wiles (Rathbone 46), Osei-Tutu (Miller 84); Smith, Kayode (Ladapo 57)
Booked: Lindsay, Barlaser, Mattock
Subs not used: Chapman, Harding, Ferguson, Edmonds-Green
Referee: David Rock
Attendance: 14,154 (301 away fans)