The two clubs have finally reached agreement after several weeks of discussion and that the 25-year-old Belgium international will head for Merseyside next week to undergo a medical and discuss personal terms.
Mignolet joined the Black Cats from Sint-Truiden three summers ago for just £2m and while he has two years remaining on his contract, his departure has become inevitable since Liverpool launched a concerted bid to lure him to Anfield.
Mignolet's arrival would push the future of Liverpool's established No 1 Jose Reina further into the spotlight.
Speculation has widely linked the Spain international with a move to former club Barcelona but, with their goalkeeper Victor Valdes insistent he will see out the remaining year of his contract, there may not be an opening at the Nou Camp.
Reina said on Thursday that he planned to see out the three years of his contract – and possibly extend it – but did concede a move to a bigger club would be a "good opportunity".
No approach has so far been received from the Catalans for Reina and, although there is still more than two months before the transfer window closes, Liverpool could conceivably begin the season with two high-profile goalkeepers on their books and both wanting to be first-choice in a World Cup year.
Manager Brendan Rodgers has said he wants competition in every position throughout his squad and with all the speculation surrounding Reina, who is currently in action at the Confederations Cup, it would have been negligent of him not to have lined up an alternative.
But it is unclear whether Rodgers wants to start next season with a situation where he has to upset either one of his most experienced – and highest-earning – players or his new signing.
Sevilla midfielder Luis Alberto is expected to be confirmed as Rodgers' third signing of the transfer window in the next couple of days after travelling to the club's Melwood training ground for a medical.
The 20-year-old has said he will use the success of Spaniards in the Premier League to motivate him.