In the 1999 summer, following Sampdoria's top level relegation, 37-year-old Mannini – who appeared in 510 official games for the club, netting eight times – left Genoa and signed for Nottingham Forest in the Football League Championship, reuniting at the English side with former Sampdoria teammate (and manager) David Platt. He made his official debut on 7 August 1999 in a 1–3 away loss against Ipswich Town, leaving in the following transfer window and closing out his career after a few months with his first team, amateurs Imolese Calcio 1919.