Jackett spend 39 days and five games without a win in charge of the Millers before quitting the club in late November, 2016 and got the expected verbal backlash from Rotherham fans on his first visit back.
Rotherham clinched their first double of the season in stoppage time when left-back Joe Mattock headed home at the far post, leaving Millers fans to sing: “Kenny, Kenny, what’s the score?”
They left new signing and 6ft 4in frontman Michael Smith on the bench, partnering Jerry Yates and David Ball up front in a 4-4-2 formation.
There was a pre-match minute’s applause for the renowned Cyrille Regis, who made his West Brom debut against the Millers, and two former Rotherham heroes, John McGlashan and Rodney Fern, who also died recently.
Rotherham did the early pressing but a lovely counter by Pompey resulted in Kal Naismith volleying wide before a drive from Jamal Lowe was parried for a corner by Marek Rodak.
The Rotherham goalkeeper also held a close-range header from Brett Pitman before Ball’s shot on the turn was deflected over at the other end.
It remained goalless and bleak at the break.
Will Vaulks twice went close straight after the break, having his header from a corner blocked and his follow-up diverted over for another flag kick, from which Richard Wood sliced wide following a scramble.
Semi Ajayi hit over after Anthony Forde won another corner as Rotherham continued to press, though Rodak was forced to punch clear from another Pompey counter.
Matty Kennedy also weaved past three men before hitting over into the 1,471-strong away support.
Joe Newell and Smith replaced Ryan Wiliams and Yates in the 65th minute and Pompey brought on Oliver Hawkins for Pitman and Connor Ronan for Sylvain Deslandes a minute later.
Former Pompey loanee Smith soon used his height but headed wide.
The Millers had forced 10 corners within 75 minutes but staunch defending kept them at bay, Jon Taylor given a late run in a bid to prise them open.
Eventually, the pressure told, Newell moving the ball out to Smith, whose deep cross was met by the head of Mattock and into the net.
Rotherham United: Rodak, Emmanuel, Ajayi, Wood, Mattock, Forde (Taylor 82), Towell, Vaulks, Williams (Newell 65), Yates (Smith 65), Ball. Subs Not Used: Price, Clarke-Harris, Ihiekwe, Cummings.
Portsmouth: McGee, Thompson, Clarke, Burgess, Deslandes (Ronan 66), Close, Donohue, Lowe, Naismith, Kennedy, Pitman (Hawkins 66). Subs Not Used: Chaplin, Bennett, May, Bass, Widdrington.
Referee: Rob Jones (Northumberland).