The England midfielder scored his third goal of the season to put the Blues top of the Premier league, but Pompey fought all the way and were unlucky not to come away with a point.
Chelsea signalled their intent from the first minute to extend their unbeaten home record to 65 matches, when good approach play saw striker Claudio Pizarro strike a low right-footer from the edge of the box, but wide of David James' goal.
Nwankwo Kanu, on nine minute, produced a speculative long-range effort that failed to trouble Petr Cech.
Florent Malouda inched closer to goal with a daisy cutter from the left but the ball hit the side netting.
But the Londoners should have taken the lead on 12 minutes when Malouda's right-wing free-kick was met by the glancing header of Pizarro from six yards out. But the ball went over the bar.
Michael Essien was called on to make a fierce last-ditch challenge on Kanu on 20 minutes to prevent the visitors from taking the lead.
The deadlock was eventually broken on 31 minutes when Lampard latched onto a deft back-heal from Didier Drogba and rammed home a piledriver under the body of James from 15 yards out.
Shaun Wright-Phillips could have had another within a minute, but James managed to get hold of the England wingman's shot from the edge of the area.
Lampard almost scored with a quick-thinking long-range effort on 44 minutes but James was alert to the strike.
Pompey narrowly missed an unlikely equalier seconds later when Matt Taylor's audacious chip from 20 yards faded away from Cech's right post.
Wright-Phillips should have scored Chelsea second goal a minute into the restart.
Malouda passed to Drogba, who fed Wright-Phillips inside Portsmouth's penalty area. The wingman got past Gary O'Neil but his right-footer was well wide of the target.
Sean Davis had a glorious chance to level on 54 minutes when Kanu held up the ball for the in-running midfielder, who lashed at it first time and sent it over the bar.
Harry Redknapp changed Portsmouth's tactics to bring on a third striker in Benjani Mwaruwari for O'Neil on 60 minutes. Mourinho responded in kind by replacing Pizarro with Salomon Kalou.
Brazilian full-back Juliano Belletti joined the fray on 65 minutes to make his Chelsea debut at the expense of Jon Obi Mikel.
An Essien piledriver forced a fine save from James on 73 minutes. And with the England management duo of Steve McClaren and Terry Venables watching, he must be in with a shout for a full England recall.
James was again called into action five minutes from time to smother a left-footed Drogba effort.
Ashley Cole spared Chelsea's blushes late on by heading a powerful header from Hermann Hreidarsson off the line.
Portsmouth continued to push for the equaliser but the Blues held out.