United have not disclosed the fee for Hernandez, but he is believed to have cost somewhere in the region of £6-8m.
He will officially join the Red Devils on 1 July.
The diminutive forward - affectionately known in Mexico as 'Chicharito', which roughly translates as 'little sweet pea' - scored in Mexico's 2-1 friendly defeat to the Netherlands in Freiburg, Germany on Wednesday.
He also featured in the second half of his country's previous friendly, the 3-1 reverse at the hands of England at Wembley on Monday.
Pacey with what looks like an good footballing head on those tiny shoulders SalfordStoo When the deal was initially struck, United manager Sir Alex Ferguson described Hernandez as "an exciting young striker".
"He has been in prolific form for both his club and his country," Ferguson added at the time. "He will be a great addition to our squad."
Hernandez is scheduled to join Ferguson's squad in mid-July for the Philadelphia leg of United's pre-season tour of the United States and Canada.
And on 30 July he is scheduled to appear against his former club Chivas in a match to open their new 45,000-seat stadium.