The Foxes, fancied for promotion at the start of the season following heavy investment under Sven-Goran Eriksson, have finished 2011 on a five-game winless run and will begin the new year languishing in mid-table.
They needed an equalising goal from former Pompey striker David Nugent to rescue a point after Marko Futacs had given the visitors the lead at the King Power Stadium.
Managers Nigel Pearson and Michael Appleton, who worked together as part of the coaching staff at West Brom, each made two changes from the sides they picked on Boxing Day.
Leicester boss Pearson opted for a 4-3-3 formation, bringing Andy King back into the midfield in place of Lloyd Dyer and selecting teenage striker Jeffrey Schlupp ahead of Jermaine Beckford.
Appleton handed Hungarian striker Futacs his first start for the club and recalled Aaron Mokoena. Ricardo Rocha and Erik Huseklepp were the players to make way.
Schlupp provided the highlight of a slow start to the game. The Ghana-born Germany Under-19 international sent a left-foot shot just wide following a driving run goalwards after three minutes.
The match then sprang into life.
Portsmouth went ahead in the 20th minute after good work down the left. The ball broke fortuitously to Futacs from David Norris' cross but the finish was emphatic as the former Werder Bremen trainee belted the ball past Kasper Schmeichel and into the roof of the net from 10 yards out.
The lead lasted just four minutes though. Nugent got in front of Greg Halford and used the pace on Paul Gallacher's superbly taken free-kick into the box to direct his header into the far corner.
Halford almost atoned for his error two minutes later when Schmeichel misjudged Joel Ward's cross from the right but the defender, arriving behind the City goalkeeper, headed against the post.
Schmeichel got down to his right to turn a low shot from Ward round the post seven minutes before the break.
At the other end, Stephen Henderson palmed Yuki Abe's long-range shot into the path of two Leicester attackers. But Schlupp and Richie Wellens got in each other's way and Henderson was able to make a block before falling on the ball and smothering the danger.
Unlike the start of the game, the second half began with a flurry of action at both ends.
Schmeichel saved a Liam Lawrence effort just 15 seconds after the restart before King stung Henderson's palms with a shot on the turn immediately afterwards.
Norris should then have done better than slice wide when he was teed-up on the edge of the area a minute later.
There was then a lengthy lull in proceedings until the 75th minute when Schmeichel, who appeared to be carrying an injury, misjudged a cross for the second time in the match and Dave Kitson headed wide.
Soon afterwards, Halford sent a header wide from a Lawrence corner.
Henderson made a double save in the 88th minute to preserve Portsmouth's point, blocking an effort from substitute Beckford before falling on the ball after King had followed up.
Leicester felt they should have had a penalty in the final minute when former Fox Joe Mattock appeared to trip Beckford but referee Colin Webster waved away the appeals.