The 25-year old Spanish midfielder will undergo a medical on Thursday before signing a four-and-a-half-year deal with the reigning Premier League champions.
Mata joined the Blues from Valencia for £23.5million in August 2011 and was voted the west London clubs Player of the Year in 2012 and 2013.
However, the Spain international has seen much less game time under Jose Mourinho since his arrival in the summer and is set to depart Stamford Bridge after making 13 Premier League appearances this season.
In total, Mata has appeared 135 times in all competitions for the Blues and has scored 32 goals including 17 goals in 82 Premier League outings.
The £37m United will spend on Mata will surpass the £30.7m they paid Tottenham for Dimitar Berbatov in 2008.
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