The veteran shot-stopper joined The Potters on a month's loan in March, and had been set to return to The Riverside but will now stay with City.
Stoke's director of football John Rudge reserved special praise for Boro, for allowing the Welshman to stay at the Britannia and play first-team football.
Whilst Crossley has been between the sticks for Tony Pulis' team, the side have kept four clean sheets in their last five matches.
"Steve McClaren has done us a tremendous favour by allowing Mark to remain on loan and we appreciate the gesture," said Rudge.
"Mark's presence has been tremendous for us - you can see that in the number of clean sheets we have kept since his arrival.
"It's good for us and good for Mark that he stays on. He wants to keep playing first-team football because it improves his chances of getting back into the Welsh team."
