Bornauw made his professional debut for Anderlecht in a 4–1 Belgian Pro League win over Kortrijk on 28 July 2018, at the age of 19 years old. He played the full game.[2] Bornauw would go on to become a key defender for Anderlecht, despite his young age. In total, he played in 24 league games and he scored one goal as a defender for Anderlecht in his debut season.
1. FC Köln
On 6 August 2019, Bornauw joined 1. FC Köln on a five-year deal.[3] He scored his first Bundesliga goal on 20 October 2019 against SC Paderborn. In his first season at 1. FC Köln, Bornauw established himself as a regular starter[4][5] and became a key player for the club.[6]
VfL Wolfsburg
On 16 July 2021, Köln announced that the club had transferred Bornauw to VfL Wolfsburg, where he signed a contract until 2026.[7]
He scored his first goal for Wolfsburg on 26 February 2022 in a 2–2 draw against Borussia Mönchengladbach.