A goal in each half was enough to sink Millwall and secure a fifth straight win for Gus Poyet's slick side.
However, they were indebted to a stunning late save from former Manchester United keeper Tomasz Kuszczak to secure victory after 10-man Millwall threatened an unlikely comeback.
Recalled striker Barnes tucked away his third goal of the season and centre-half El-Abd grabbed his first in nearly two years.
But Chris Wood pulled one back on his full debut and the on-loan West Brom striker almost made himself an instant hero in stoppage time only to be denied by Kuszczak.
The Lions also had winger Chris Taylor sent off and a third straight defeat left them in the bottom three.
Wood almost made his mark after just five minutes with a powerful header, but Kuszczak made a smart save.
And Brighton were soon on top with Wayne Bridge forcing Lions keeper David Forde into a fingertip save with a low, angled drive before Will Buckley fired narrowly wide.
Brighton's pressure told in the 17th minute when Buckley beat Scott Malone to the byline and his cutback was expertly sidefooted into the net by Barnes from 10 yards.
The Seagulls should have gone two up after half an hour when Forde, back in the side at the expense of Maik Taylor, came racing out to collect a long ball only for Buckley to get there first.
The winger nodded the ball past Forde but by the time he steered his shot towards the net, Shane Lowry had got back to block on the line.
Buoyed by that let-off, Millwall improved as the first half drew towards a close but Kuszczak was not unduly troubled as he saved efforts from James Henry and Liam Trotter while Wood blazed Nadjim Abdou's cross over the bar.
Instead Brighton did double their advantage at the start of the second half when Millwall failed to deal with Andrea Orlandi's corner and the ball fell to El-Abd, who swept it home from six yards.
Barnes had a header cleared off the line by Trotter following another poorly-defended corner as Brighton looked to kill the Lions off.
And the hosts' misery was compounded when substitute Taylor left a foot in when he went in for a 50-50 ball with Kuszczak, leaving the Brighton keeper in a heap and referee Andre Marriner waving a red card.
Yet Wood's drive 11 minutes from time gave the 10 men hope and, in stoppage time, the ball fell to the young New Zealander again only for Kuszczak to block his point-blank volley to ensure Brighton took top spot.