Having been held 2-2 by Juventus in their Group E opener, Chelsea's campaign is now up and running, but the scoreline disguised the fact that Roberto Di Matteo's side toiled for long periods at Copenhagen's Parken Stadium.
Making their home debuts in the tournament – albeit away from their Farum Park home – Nordsjaelland took the game to the visitors after Mata's 33rd-minute opener and threatened to equalise several times before David Luiz made the game safe 11 minutes from time.
Mata and Ramires added late goals to rubber-stamp Chelsea's first away win in the group phase since a 2-0 success at Spartak Moscow in October 2010.
"It was a great result," said the recalled Frank Lampard.
"We had to bide our time. At 1-0 they created a few half-chances and hit the post but once we got the second goal, that was it and we dominated," he told ITV.
With four points, Chelsea are now joint-top of the group with Shakhtar Donetsk, who they visit in their next match.
Chelsea made three changes to the team that won 2-1 at Arsenal on Saturday, Lampard coming into the team alongside Victor Moses and Gary Cahill, who replaced John Terry despite the Blues captain having served a two-game Uefa ban.
Moses was making his competition debut and he should have marked it with an early goal, but when Oscar's dink found him at the back post in the fourth minute, he planted the ball against the upright from point-blank range.
Lampard fired over from a Mata cut-back shortly afterwards, but the hosts did not appear inhibited by the stature of their opponents.
Nordsjaelland coach Kasper Hjulmand had asked his players to show "courage" and they displayed impressive assurance on the ball.
Their ease in possession was to prove a double-edged sword and after Fernando Torres had tested goalkeeper Jesper Hansen in the 22nd minute, the visitors exploited that over-confidence and took the lead.
Enoch Adu was at fault, the centre-back dwelling on the ball and being robbed by Torres, whose pass was helped into the path of the onrushing Mata by Lampard, enabling the Spaniard to finish with the inside of his left foot.
"We made a stupid mistake on the first goal but then we came back and we tried to get possession of the ball and play through their lines," said Hjulmand.
"Many of the things we've been practising succeeded, but we needed to put our chances away."
The Danes responded with a low shot from Patrick Mtiliga that Petr Cech fielded comfortably, before Torres spurned a good chance to make it 2-0 when he shot at Hansen after Oscar's pass caught the home defence square.
Chelsea had the lead but they failed to seize the initiative and the Premier League leaders were subjected to three scares in the first 15 minutes of the second half.
Joshua John shot wide of the near post after cutting in from the left flank, and after Mikkel Beckmann looped a header onto the roof of the net, Cech had to beat away a firmly hit drive from home skipper Nicolai Stokholm.
They were to come even closer in the 73rd minute. Once again, John danced infield from the left, this time dispatching a curler that Cech touched onto the far post, with Beckmann's follow-up blocked by Ashley Cole.
Only in the closing stages did deliverance arrive for Chelsea, after Luiz slammed a 25-yard free-kick past Hansen in the 79th minute.
Mata claimed his second three minutes later, finding the bottom-right corner after darting into the box from the right, before Ramires applied the coup de grace with a tap-in from Oscar's pass. – Sapa-AFP