Mitchell has been absent from the Pools squad since the New Year after his short-term contract expired and the former Newcastle United stopper will become a free agent having opted against signing a new contract with the club.
Mitchell arrived from Derby County in the summer but has struggled to force his way into the starting line-up at the Suit Direct Stadium under both Lee and previous manager Dave Challinor.
Mitchell has been limited to appearances in the cup competitions with Ben Killip taking over the gloves for League Two duty.
The ex-Magpies stopper spent last season on-loan at Northampton Town where he regularly featured in League One.
Pools boss Graeme Lee revealed earlier this month the club had offered Mitchell a new deal to remain at the Suit Direct Stadium beyond his three month contract but it is an option the goalkeeper has opted against.
“I don’t think Jonathan will be coming back,” Lee said after Pools drew 0-0 with Carlisle United on Tuesday.
“We spoke to him about giving him [a new deal] and we offered him something but obviously it didn’t suit him so we’re speaking to other keepers.”
Pools had been close to bringing in ex-Sunderland goalkeeper Trevor Carson before the 33-year-old signed for League One side Morecambe.
Pools have had several goalkeepers on trial at their Maiden Castle training base over the last few weeks in the event Mitchell would not be returning.
And now Lee must accept he will need to find a new goalkeeper over the remaining two weeks of the transfer window with youngster Patrick Boyes the only other registered goalkeeper behind Killip.
Pools have also announced that defender Jake Lawlor’s deal with the club has been terminated by mutual consent.
Lawlor joined the club after a spell with Harrogate Town but made just two appearances before being sent out on-loan to National League side Bromley.
The defender also spent time on-loan with Pools’ rivals Darlington this season but now leaves the club via a mutual agreement.