The powerfully-built 25-year-old renews a successful acquaintance with Mowbray, having enjoyed the most successful spell of his career to date when the Boro manager was his boss at West Brom.
Miller, who is 6ft 3ins tall, began his career with hometown club Manchester City but his career took off after he joined up with Mowbray on loan at The Hawthorns in July 2007. He scored seven goals in 21 league games and was called into the England U-21squad.
His form led to a move to the Baggies on a permanent basis for £1.4m in the January transfer window that season and now he's hoping to repeat that success with Boro.
"I'm very happy and excited to be reunited with Tony Mowbray and Mark Venus," Miller told mfc.co.uk this afternoon.
"Tony gave me my first shot of playing regular first-team football and I will always be grateful to him for that.
"Playing for Tony was the best time of my career so far."
Miller spent 14 months on the sidelines with a cruciate knee ligament injury sustained after a clash with Portsmouth goalkeeper David James in December 2008. He later went on loan to Queens Park Rangers, helping Neil Warnock's side win the Championship title.
He moved on to Forest for an initial fee of £1.2m in August 2011 and he scored four goals in 13 starts for the Midlands club last season.
"I have had a few injuries but hopefully they are now all behind me. I'm feeling great and feeling strong," Miller said.
"I'm just relishing the opportunity to play for Boro. I was excited when I heard Tony Mowbray was interested in me. I had a few different options, but Tony knows what I can do and I really enjoy working with him.
"I like the style of football his teams play and he also made me a better player. I would have liked to work with him for longer and now I have that opportunity.
"I had ups and downs with Forest but I am very grateful to them. They are a massive club with a great fan base and wish them all the best.
"Tony Mowbray is clearly stamping his own authority on the team and hopefully I can show the fans what I can do."