He now joins up with former Reds Stephane Henchoz and David Thompson, who has also joined Paul Jewell's side in this transfer window.
"He will bring us a new dimension, and add competition to the squad which we need at the present time," said the Wigan boss.
"Neil is a good young striker who is looking for a chance to prove himself again at this level. He is strong and skilful with an eye for goal. I'm delighted to have him on board."
Mellor will be unable to play against Liverpool on February 11th because of Premier League rules.