Jack made his senior debut for Brommapojkarna at the age of 15 in a Svenska Cupen game against Norrköping in March 2017.[1][2] Lahne made his Superettan debut for Brommapojkarna during Summer 2017.[2] He scored his first goal for Brommapojkarna in a Superettan game against Åtvidaberg in July 2017, making him Brommapojkarna's youngest ever goalscorer at the age of 15 years and 271 days.[2] At the end of the 2017 Superettan season, Brommapojkarna won promotion to Allsvenskan.[3]
During the 2018 Allsvenskan season, Jack Lahne scored 4 goals in 19 games as Brommapojkarna were relegated back to Superettan after losing in the relegation playoffs against AFC Eskilstuna, in which Lahne scored one goal.[4]
Amiens
In January 2019, Lahne joined Amiens on loan until the end of the 2018–19 Ligue 1 season.[5] On deadline day, 4 April 2019, however, the loan deal was converted to a permanent transfer, before Lahne had had the chance to play for Amiens.[6]
Lahne scored his first Ligue 1 goal for Amiens in a game against Montpellier on 30 November 2019, which was also his debut.[7]
Loan to AIK
On 4 April 2019, the same day SC Amiens purchased Lahne, it was announced the club had agreed to loan Lahne to reigning Allsvenskan champions AIK until 31 July 2019.[6][8] Lahne made his Allsvenskan debut for AIK on 8 April 2019, coming on as a substitute for Saku Ylätupa in a 0–0 draw against IFK Norrköping.[9] On 14 April 2019 in an Allsvenskan game against IK Sirius, Lahne scored his first goal for AIK in a 2–1 win.[10] On 1 July 2019, Amiens exercised their option to end Lahne's loan deal with AIK prematurely.[11]
Loan to Örebro
On 28 February 2020, Lahne joined Örebro SK on loan until the summer of 2020.[12] He made his competitive debut for Örebro in a 2019–20 Svenska Cupen game against IF Elfsborg in February 2020.[13] He played in 10 Allsvenskan games without scoring before returning to Amiens on 31 July 2020.[14]