Ancelotti's men capitalised on a rare slip-up from Barca, who lost 1-0 at home to Malaga on Saturday, with a routine victory at the Martinez Valero in the Italian's 100th game in charge of Real.
Karim Benzema - who had earlier seen an acrobatic effort wrongly disallowed for offside - opened the scoring with his 18th goal of the season from close range in the 56th minute.
Cristiano Ronaldo then headed home a second 13 minutes later to secure a result that cemented Real's grip on top spot.
Despite their advantage, though, Ancelotti is refusing to get complacent about Real's title prospects.
"It was an important game after what happened yesterday [Saturday], above all to take a distance over Barcelona," Ancelotti said.
"It is clear that yesterday's defeat surprised everyone, as Barca had been in very good form, playing very well.
"[It is] clear we have a lead now, but as I have said many times this title will be decided on the last day of the season."
Ancelotti's time in charge at The Santiago Bernabeu has seen him win the UEFA Champions League, FIFA Club World Cup, the Copa del Rey and the UEFA Super Cup.
And, after marking his century with three points, Ancelotti delivered a modest assessment of his spell at the helm of the capital club.
"The first 100 have gone well," Ancelotti added. "And I hope the next 100 do too.”
