A superb display from Roberto Di Matteo's team was rewarded with the Ivorian's goal 15 minutes from time, after Chelsea had survived a couple of scares and also hit the post through Juan Mata.
Di Matteo made six changes from the side which was held to a goalless draw at home by Tottenham at the weekend, one of which was the inclusion of Fernando Torres instead of Didier Drogba as Chelsea's lone striker.
The Spaniard led the line well for the Blues in the first half and was to prove influential later on, as he linked well with Kalou and Mata throughout the evening, causing the Portuguese side problems with his movement and willingness to pull wide to receive the ball.
Paulo Ferreira replaced Jose Bosingwa at right-back in the starting XI and there was a nervy moment for the defender early on when he swiped at and missed a free-kick into Chelsea's area, and as impressive as the visitors were going forward, Benfica were no less dangerous when allowed inside Chelsea's half.
The first real chance of the game came 20 minutes in, and despite Chelsea's better work-rate up front it fell to Benfica. Bruno Cesar fed Oscar Cardozo who, one step ahead of John Terry, brought the ball down on his chest and half-volleyed wide.
Torres continued to plough the line up front, turning and volleying well over in what was a confident attempt on goal, and David Luiz showed composure to juggle the ball inside his own six-yard area before clearing as each side sought a clear-cut opening.
Bruno Cesar had a solid if unchallenging shot from distance saved by Petr Cech, but it was the Chelsea player's opposite number Artur who had to really work when, after some excellent build-up play between Torres and Kalou, Raul Meireles let fly with a fierce low shot which the Brazilian goalkeeper had to get down low to his left to turn round the post.
Starting the second half strongly Benfica had a Cardozo shot blocked by David Luiz on the line before shouts for a penalty were turned away when Maxi Pereira fired the ball into Terry's arm, but it would have been a harsh spot-kick to give.
Chelsea went agonisingly close on the hour mark when Mata latched onto a long ball, rounded Artur and struck the outside of the post from a very tight angle, but shortly after they could just have easily been a goal down when Jardel had a free header well saved by Cech. Moments later Terry got a crucial toe on a through ball from substitute Rodrigo with Cardozo lurking threateningly behind him.
Chelsea's goal came on the counter and it was a superbly worked one, with Ramires winning the ball in a dogged challenge before giving it to Torres just inside Benfica's half. The Spaniard held Jardel off, raced clear down the right and picked Kalou out perfectly as the Ivorian slid in to give the Blues the lead in the 75th minute.
Benfica tried - tiredly - to manufacture an opening, substitute Nolito shooting over, and Chelsea could have dealt their hosts a major blow when substitute Daniel Sturridge broke clear: with a numerical advantage he passed to Mata, but the forward's chip was overcooked and sailed over the bar.
Chelsea held out through five minutes of added time to repel some last-ditch Benfica attacking, Ashley Cole cutting a Nicolas Gaitan cross out and narrowly avoiding putting the ball into his own net, and there were justifiably jubilant scenes from Di Matteo and his players at full time as they secured a precious away-goal advantage to take to Stamford Bridge for the return leg next Wednesday.