The 24-year-old defensive player has been under contract with Borussia Dortmund since July 2012, including some loan spells. In 2016, he made the jump from the club's academy to the senior side. As of June 2022 (reference date for all stats), he has played 49 games (3 goals, 2 assists) for BVB across all competitions and helped Dortmund on their way to winning the DFB-Pokal in 2017 and 2021.

He started playing football as a four-year-old for SV Fortuna Bottrop. He remained loyal to his hometown club for eight years before joining Rot-Weiß Oberhausen in 2010, and then, two years later, he moved to Borussia Dortmund.  He won the German Championship twice with the U18s - in 2014 and 2015 - and then repeated the feat in 2016 with the U19s. Not many players can boast of winning three titles in three years! He captained the BVB U19s in their 5-3 win over TSG Hoffenheim in the 2016 final and was involved in each of the five goals, scoring one himself.

In 2015, he was given the Fritz Walter gold medal awarded to the best young player of his year group. In the space of just six months, he represented his country at both the U17 Euros in Bulgaria and the U17 World Cup in Chile. At the former, the powerful right-footer shone as the second-highest goalscorer in the tournament, while at the latter, he impressed as captain and midfield general.

Passlack made his Bundesliga debut for the first team on 2 March 2016 in a 2-0 win away to SV Darmstadt 98. That year, his goal – the seventh in the 8-4 win over Legia Warsaw in November 2016 – made him the youngest-ever German goalscorer in the UEFA Champions League, at 18 years and 177 days old. He spent the 2017/18 season out on loan at TSG Hoffenheim, the 2018/19 season at English club Norwich City (helping them win promotion to the Premier League) and the 2019/20 season at Fortuna Sittard in the Netherlands, where he played in 25 of their 26 Eredivisie matches before the cancellation of the season due to the Covid-19 pandemic.

Felix Passlack spent three years searching for happiness in football, at three different clubs. He finally found it back where it all began – at BVB. "I couldn't have imagined it would have been much better than it has been up to now," he told the BORUSSIA members' magazine in December 2020, adding: "I told myself: 'This is your last chance, so you have to give everything, prove yourself'. I am glad that the coach and the people at the club have put their trust in me." As a flexible player whose qualities shine through on both ends of the pitch, he offers something different and vital to the squad.

In 2021/22, he played in 14 of his team's 46 competitive matches. In the Bundesliga, he started five games and also came on as a substitute five times. On Matchday 1, in the 5-2 home victory against Frankfurt, he scored the first own goal of his Bundesliga career – through no fault of his own. On Matchday 33 in Fürth, he managed to score at the right end – his second Bundesliga goal after having found the net at home against Freiburg in October 2020.