The centre-back, 26, has become a key target for new Forest manager Stuart Pearce, who made Mancienne his captain when manager of the England Under-21s.
And Forest are believed to have agreed a fee of around £1million to bring him to the City Ground.
Mancienne began his career as a trainee at Chelsea and enjoyed successful loan spells at QPR and Wolves before he was signed by former Chelsea sporting director Frank Arnesen at Hamburg in 2011.
After a promising start in the Bundesliga, Mancienne struggled to hold down a first-team place and made just 14 appearances for the club last season, including two for Hamburg II in the fourth tier of German football.
And Pearce, who played Mancienne 30 times for England’s U21s, is looking to revive the defender’s career as he looks to take Forest back into the Premier League this season.
Pearce, who took over at the end of last season, has already captured a number of promising signings in the current window.
Former Hull City striker Matty Fryatt, young defender Louis Lang and Barcelona academy graduate Roger Riera have all signed on free transfers, while exciting Dutch forward Lars Veldwijk has joined from Excelsior for an undisclosed fee.