The 25-year-old Spaniard, who joined United from Chelsea in January, scored twice, including directly from a free kick, to take his tally to three goals in his last two games.
“I think he has made us better,” United manager David Moyes said. “He hasn’t had the goals in the earlier games, but I really thought he played fine in the games.
“But recently he has started the get the goals and, more importantly, assists as well. Over the last few seasons in the Premier League, Juan Mata is one of the best assist-makers and he has always got himself a good tally of goals.”
Showing that United can cope without injured striker Wayne Rooney and Robin van Persie, Javier Hernandez and substitute Adnan Januzaj added further goals in the second half.
Moyes‘ United moved up to sixth place, while Newcastle stayed ninth. The priority for United is to follow up a 1-1 home draw with Bayern Munich with a victory in Germany on Wednesday to reach the Champions League semifinals.
Moyes made wholesale changes for the trip to Newcastle with only three of the players who started against Bayern Munich in midweek - Phil Jones, Antonio Valencia and Marouane Fellaini - retained with one eye on Munich.
However, once Mata had fired them ahead from a 39th-minute free kick which Magpies manager Alan Pardew felt should never have been awarded, United never looked back.
Mata added a second from close range five minutes after the restart and Javier Hernandez put the result beyond doubt 15 minutes later, before substitute Januzaj wrapped up a comfortable win with the help of a deft assist by the Spain international.
It handed Newcastle a seventh loss in 10 league games.
“It’s tough. I understand the fans’ frustration,” manager Alan Pardew said. “They want to see exciting football at this stadium and we are just not producing it at the moment.”