 He attended Cranleigh School as a youngster and during his time in Harlequins academy, he spent several games playing for Esher RFC.Stegmann scored his first try for Quins 1st XV on 22 March 2009 against Sale Sharks.On 25 February 2013, Sebastian Stegmann would leave Harlequins to join London Welsh on a two-year contract for the 2013/14 season.