The Lions forged ahead in the 15th minute on Saturday when home captain Richard Naylor scored an own goal in front of a stunned Kop.
Yet Simon Grayson's men responded in style and Lloyd Sam drew them level in the 32nd minute before Somma struck twice in clinical fashion late on to seal a win that was no more than Leeds deserved.
The woodwork saved Millwall on three occasions but Somma stuck within four minutes of his arrival and netted again in added time to rock the visitors.
Kenny Jackett's men arrived as league leaders after impressive 3-0 and 4-0 wins over Bristol City and Hull, but Leeds offered the greater goal threat for much of the game and nobody impressed more than Sanchez Watt.
The on-loan Arsenal winger, 19, was pivotal throughout and had already had three attempts on goal by the time Millwall hit Leeds with a sucker-punch.
Steve Morison's high cross from the right was hopeful at best but Naylor, under minimal pressure from Kevin Lisbie, headed the ball straight into the top corner past Kasper Schmeichel.
Yet Leeds fashioned an impressive response, with Watt and homegrown midfielder Jonathan Howson at the heart of their best moves.
In the 23rd minute, Howson teed up Luciano Becchio for a shooting chance from 10 yards but the Argentinian could only blaze wildly over the crossbar.
Three minutes later Howson burst into the box and showed some delightful footwork to engineer the space to rifle a low shot against the outside of a post as Millwall survived.
The Lions' goal began to lead a charmed existence and after Sam had a shot cleared off the line in he 27th minute, he drew Leeds level five minutes later.
Watt drifted across the face of Millwall's penalty box before finding Becchio.
He kept the move flowing with a neat touch into the path of Sam, who struck a fierce low right-footed shot past Forde and into the bottom right-hand corner.
The goal poured confidence into Leeds and they could have been comfortably ahead by half-time.
Defender Neil Collins headed wide when he should have hit the target, Watt shot weakly wide at Forde when well placed, and Forde then produced a fine save to tip Howson's 20-yard strike around a post.
From the resultant corner by Neil Kilkenny, Collins headed the ball against a post as Millwall were again rescued by the woodwork.
Millwall improved vastly after the break but genuine chances were scarce for both teams.
Leeds always looked the more likely to claim maximum points and so it proved as Somma struck with 11 minutes remaining.
More intelligent play by Watt saw him drive a low cross across the face of goal and Somma was perfectly placed to smash the ball into the net from 10 yards.
Schmeichel then produced a fine stop at the other end to preserve Leeds' advantage before Somma reclaimed centre stage in added time.
Somma showed brilliant footwork to engineer the space to hit a superb left-footed shot past Forde from 12 yards.