Mowbray's Boro dominated against their rock bottom visitors and looked home and dry after skipper Matthew Bates finally fired then ahead in the 70th minute.
But Billy Jones scrambled a late equaliser for Preston, bundling the ball over the line to secure a 1-1 draw at the Riverside Stadium.
"That hurts," said Mowbray, whose team have now gone five matches unbeaten in the league. "It feels like a defeat - worse if that's possible.
"It is bitterly disappointing because that second half was as good as we have played since I came here and we had two really good chances to kill the game
"We had an excellent second half and that is exactly how we want to be playing as a team, that is what we are striving for. But that sounds hollow because at the end of the day we have only taken one point from a team we really should have beaten."