Lawrence hit his fifth goal of the season with 12 minutes remaining as the south coast club extended their unbeaten run to five matches.
They also had the woodwork to thank after Chris Hackett hit a post and Alan Dunne rattled the crossbar.
But things are looking up both on and off the field for Pompey, who expect to finally exit administration later this week.
Boss Steve Cotterill even had the rare luxury of being able to tinker with his starting line-up, with new loanee Greg Halford making his debut after joining from Wolves.
But it was Millwall who made the quicker start with Wales striker Steve Morison firing narrowly wide in the opening seconds.
Pompey then failed to clear Hackett's free-kick and Darren Carter's low shot, through a crowd of players, was beaten away by keeper Jamie Ashdown.
Pompey finally registered a shot on goal half an hour in, John Utaka beating two defenders on the edge of the box before shooting too close to David Forde.
But the visitors, still only able to name five subs, were forced into an unwanted reshuffle moments later when Ricardo Rocha limped off to be replaced by youngster Joel Ward.
They finished the first half strongly, though, with David Nugent shooting wide and Lawrence curling a free-kick narrowly over the top.
Nugent should have put them ahead in stoppage time when he went charging through but he delayed his shot allowing Darren Ward to clear.
Nugent had another gilt-edged opportunity straight after the interval when Utaka reached the byline and cut the ball back, but it got stuck under the former England frontman's feet and the chance went begging.
Moments later Nugent was able to tee up Utaka but this time the Nigerian's shot was high and wide.
Pompey survived a major scare in the 53rd minute when Scott Barron's free-kick across the box fell to Hackett.
His drive took a couple of deflections and eventually came back off the foot of the post.
And Carter should have done better when Liam Trotter's curling pass landed at his feet on the edge of the six-yard area but he blazed over.
Substitute Dunne was next to try his luck for the Lions but his 20-yard strike grazed the top of Ashdown's crossbar.
But it was Pompey who had the last word after Nugent was tripped by Hackett and Lawrence rammed home the penalty to seal the points.