Mowbray decided to swoop for the 26-year-old former Sunderland and Ipswich midfielder after holding talks with the free agent during the summer, and has been richly rewarded for his faith in the player with Leadbitter having started each of Boro's 17 Championship fixtures to date this season.
Mowbray said: "He had some choices to make in the summer and great credit to him, he wanted to come to a club that played a brand of football he enjoyed.
"I had a few conversations with him and nothing has changed my mind about the opinion I formed of him then.
"He loves football, sets very high standards and likes things done properly. He knows what's right and what's wrong and he has an opinion.
"I'm very happy he's here helping us achieve what we can this season."