Humphrey is a free agent after seeing out his contract at Fir Park and a number of clubs have expressed an interest in his signature.
Rangers and several Championship sides have been linked with signing the flying winger, but Preston appear to have won the battle for the Jamaica international.
Humphrey is believed to be undergoing a medical at Deepdale ahead of tying up his move to Simon Grayson's side.
The 25-year-old is expected to sign a three-year deal at Preston and his signing will be a boost for Grayson as he looks to mount a bid for promotion to the Championship next season.
Sky Sports also understand that Humphrey's capture will be followed up by the news that Preston have also tied attacking starlet Will Hayhurst down to a new long-term deal.