Nugent's 11th league goal of the season provided Portsmouth with a welcome away win which moves them to a point behind Leicester in mid-table. With eight games remaining, Leicester stand five points outside the play-offs.
Sven-Goran Eriksson's home side dominated for large periods of the game but could not find an equaliser as a dogged defensive performance from Portsmouth saw Steve Cotterill's troops claim their eighth clean sheet in nine matches.
Leicester started the brighter of the two teams as Hermann Hreidarsson almost turned Richie Wellens' cross into his own net inside the first five minutes.
Former Portsmouth striker Ayegbeni Yakubu should have put the Foxes ahead after 10 minutes but he headed Paul Gallagher's teasing cross wide from six yards.
The visitors opened the scoring against the run of play after 13 minutes when Souleymane Bamba's poor defensive header landed at the feet of Nugent and the striker made no mistake, firing into the bottom corner from 12 yards.
Bamba then almost redeemed himself but the Ivorian's header was parried away by Jamie Ashdown while Gallagher went close, shooting wide from 25 yards, as the home side looked for an equaliser.
Liam Lawrence, restored to Cotterill's starting line-up after recovering from injury, cracked a 25-yard free-kick just wide of Ricardo's goal before Gallagher failed to direct another shot on target after smart work from Yakubu.
Leicester had an appeal for a penalty waved away by referee Paul Tierney on the stroke of half-time as Yakubu went over easily, while Bamba sent a header off target.
Eriksson's side were infuriated after 55 minutes when Gallagher's free-kick looked to have hit a visiting player's hand but again Tierney waved away the protests.
The dangerous Yakubu shot wide from long distance as Leicester took the game to the visitors and Gallagher wasted another opportunity when one-on-one with Ashdown, with the goalkeeper keeping his side ahead with a smart save.
Eriksson threw on Lloyd Dyer and Martyn Waghorn to try to find an equaliser but it was Portsmouth who had a great chance 10 minutes from time. Joel Ward fluffed his finish from close range after Bamba looked for an offside flag.
Ashdown denied Leicester an equaliser after 82 minutes when he superbly parried away a long-range Kyle Naughton strike, and a superb cross from on-loan defender Jeffrey Bruma fizzed across the face of the goal without a home player managing to get anything on it.