A former pupil of St Peter's High School, he made his professional debut in the European Challenge Cupquarter final against Brive in 2006 which Gloucester Rugby won comfortably. Lamb started in the subsequent final, as Gloucester defeated his future club, London Irish. He became first choice at Fly-half for Gloucester Rugby as they topped the table during the 2006–07 Guinness Premiership and 2007–08 Guinness Premiership seasons. However, they were thwarted in the play-offs in both seasons by the Leicester Tigers. Gloucester's continued failures in key games saw Dean Ryan lose faith in Lamb. He was dropped after Gloucester's home Heineken Cup defeat to Cardiff Blues in January 2009 amid concern that he failed to exercise sufficient control over games. Lamb was released by Gloucester at the end of the 2008–09 season. Lamb signed for London Irish. During the Guinness Premiership match between London Irish and Sale Sharks on 28 March 2010, Lamb successfully converted a penalty to take his points tally for that competition past the 500-mark. On 24 March 2011, he signed a contract that took him to Northampton Saints for the 2011/12 season. On 20 February 2013, Lamb would leave Northampton Saints to join Leicester Tigers for the 2013/14 season. However, on 10 February 2014, Lamb left Leicester Tigers after seven months to join with Worcester Warriors with immediate effect, where he rejoined Director of Rugby Dean Ryan at the Sixways Stadium. He scored the winning conversion as Worcester were promoted back to the Aviva Premiership for the 2015–2016 season, beating Bristol 59–58 on aggregate. On 19 May 2017, Lamb left Worcester for France, joining the Top 14 club La Rochelle on a two-year deal ahead of the 2017–18 season.
Lamb was arrested in the early hours of 1 March 2009 outside Subtone, a nightclub in Cheltenham. Lamb was charged with assault, and was later fined for the incident. On 9 May 2010, Lamb was hit in the face with a bottle after an altercation involving fellow London Irish players and supporters of Chelsea Football Club in Clapham. He was taken to hospital but was not seriously injured. Lamb is a keen warhammer player and is currently regional champion. He boasts an impressive elven army and is unbeaten in his last five matches.
