The midfielder was a consistent target for the Reds throughout the summer with their interest continuing right up until the end of the transfer window.
Middlesbrough stood firm, however, and are looking to Leadbitter to lead a promotion challenge that manager Mowbray believes they are more than capable of.
“They (Forest) tried to sign him," he told The Northern Echo.
"Every player has his price, but we are not daft, we are not giving away our best players and they were made aware of that."