On 16 July 2014, Mancienne joined Nottingham Forest on a three-year deal, reuniting with his England U21 manager Stuart Pearce. He made his debut against Blackpool on 9 August 2014, starting at centre-half alongside Danny Fox. He played the entire ninety minutes of the 2–0 Forest win, earning the Man of the Match award in the process. Mancienne was then sent off in his second game for the club, following two bookable offences in Forest's League Cup tie away at Tranmere Rovers on 12 August 2014. Following his suspension, he returned to the side and once again was nominated the fans' Man of the Match in a 1–0 away win at Sheffield Wednesday.