Mellor has signed a three-year deal at Deepdale after agreeing personal terms and passing a medical.
The 23-year-old found himself well down the pecking order at Anfield following the arrivals of Craig Bellamy and Dirk Kuyt this summer.
Preston boss Paul Simpson was delighted to land the talented forward.
"Neil is a goalscorer and hopefully that will continue to be the case for us," Simpson told the club's official website.
"He is a tremendous signing for the club.
"We accept that he has had his injury problems but we are happy that he is over those and it's just a case now of building up his fitness levels."
Mellor's signing is sure to fuel speculation that David Nugent could be on his way out of the club.
Middlesbrough are understood to have had one bid rejected by Preston for the England Under-21 international.
The Teesside outfit are keen to land the former Bury hit-man and could be ready to offer Malcolm Christie in part-exchange in a bid to land Nugent.
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Good luck to Neil. He helped us out loads during 2005. He will go down in history for his part in the Olympiakos game. I will always remember his last minute wonder strike against Arsenal. He was never LFC standard but will be remembered. Injuries took their toll and he's gone. He will do well at Preston and I hope we see Neil at anfield playing for Preston in the premiership very soon.
Thanks Neil
YNWA
