The French centre-half thundered a header past Sergio Alvarez on 73 minutes to break the hosts' resistance at Balaidos and maintain their advantage over second-placed Los Blancos.
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Just one of two teams to beat Barca at Camp Nou this season, Celta reminded the Catalans how dangerous they can be when Joaquin Larrivey - who scored the decisive goal in Catalunya - saw his low drive pushed away by Claudio Bravo after 10 minutes.
Despite an abundance of possession, the Primera Liga leaders struggled in the final third but did come close on 13 minutes, with Neymar firing wide after Lionel Messi's curler was well saved by Alvarez.
It was the hosts who finished the half in the ascendancy. First, Eduardo Berizzo's side had a goal chalked off for offside, before Larrivey inexplicably failed to convert after being found by Fabian Orellana's cross.
With four successive Liga victories under their belt, Luis Enrique's side began finding their rhythm in the second half and Neymar's stunning effort was also ruled out for offside on 48 minutes.
Unperturbed, Barcelona carried on in the same vein as Messi perked up and fired wide, before Dani Alves spurned arguably the chance of the game after 63 minutes and missed the target from a fine position.
It was not all one-way traffic, however, as Celta found their first-half fluency at times and Hugo Mallo curled just wide on 71 minutes - a warning that the visitors finally acknowledged.
Mathieu, who opened the scoring against Real Madrid a fortnight ago, scored in back-to-back outings as he rose highest to plant a fine header past Alvarez from Xavi's free kick.
Things went from bad to worse for Celta in the dying stages as Orellana was given his marching orders for appearing to throw a piece of turf at Sergio Busquets's face as the clash ended on a sour note.