Steele got a hand to Brian Stock's deflected effort as it crept through a crowd of players on the edge of the 18-yard box, but could only palm the ball into the bottom corner of the net.
The 93rd-minute slip up cost the Teessiders a point, and provided a disappointing end to a week that has seen the 20-year-old shotstopper sign a new four-anda- half year deal at the Riverside.
Steele was all but inconsolable at the final whistle, kneeling with his head in his hands, but Mowbray was quick to express sympathy for his plight.
I feel very sorry for the young keeper, said the Middlesbrough manager, whose side will slip back into the relegation places if Preston, Scunthorpe or Crystal Palace win today. The ball has spun away from him at the last minute.
That's the kind of luck you don't want, but it's the kind of luck that seems to keep happening to us at the moment.
Jason was obviously disappointed in the dressing room, like all the players, but he's a fantastic prospect and I'm not going to be too hard on him because of what happened.
Steele's mistake provided a dramatic denouement to proceedings, but earlier misses from David Wheater and Kris Boyd were every bit as costly.
Despite James Hayter's equaliser having cancelled out Scott McDonald's fourthminute strike, Boro looked the likelier winners for the majority of an entertaining second-half at a freezing Keepmoat Stadium.
However, Wheater fired over with five minutes left when it might have been easier for him to head the ball, and substitute Boyd produced a quite staggering miss in the 89th minute.
The Scotland international was poised to side-foot home after Joe Bennett crossed from the left, but somehow missed his kick with the goal seemingly at his mercy.
I'm absolutely gutted,
said Mowbray. In the last 20 minutes, we looked like the only side that was going to win the game, but we missed a couple of very good chances and paid the price.
That's one of the main things we have to improve.
We have to be more clinical because we are passing quite a few chances up at the moment.
We have to be more clinical and make the most of the things that come our way.
The mood in the Middlesbrough dressing room was understandably sombre following the final whistle, and Mowbray now faces the difficult task of picking his players back up ahead of Christmas fixtures against Nottingham Forest and Preston.
We can't afford to feel sorry for ourselves, he said.
We've got to get back out there and make sure we start getting the right reward for our efforts. It's tough because nothing really seems to be going for us at the moment.
We've just got to keep working hard and believing in what we are doing. It's going to take time, but the players are working extremely hard and that's all they can do. It's just not going our way, but that will change.
Meanwhile, highly-rated youngster Cameron Park has signed his first professional contract and committed himself to Middlesbrough until 2015.
The Middlesbrough-born 18-year-old, who now lives in Marske, has signed a fourand- a-half year deal.