The two clubs are understood to have reached an agreement after protracted discussions and the Belgium international could now undergo a medical and discuss personal terms next week.
Mignolet, who moved to the Stadium of Light from Belgian side Sint-Truiden three years ago for £2m, still has two years remaining on his contract but Sunderland appear to have found it too hard to resist Liverpool's overtures.
A deal will increase speculation over the future of the Reds' established No 1, Pepe Reina.
The Spaniard has been linked with a summer return to Barcelona but has three years left on his own contract and has not yet been the subject of any approach.
Current Barca goalkeeper Victor Valdes remains insistent he will see out the remaining year of his contract so 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".
Although there is still more than two months before the transfer window closes, Liverpool could begin the season with two high-profile goalkeepers on their books.
That is a potentially tricky situation given that both will want 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, 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 highly-paid players or his new signing.