Jack made his senior debut for Brommapojkarna at the age of 15 in a Svenska Cupen game against Norrköping in March 2017. Lahne made his Superettan debut for Brommapojkarna during Summer 2017. 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. At the end of the 2017 Superettan season, Brommapojkarna won promotion to Allsvenskan.
