[2] A youth academy graduate of Copenhagen, Kristiansen made his senior team debut on 4 November 2020 in a 2–1 cup win against Avarta.

[6] On 20 January 2023, Kristiansen signed for Leicester City for a record-breaking fee of £17 million, making him the most expensive Danish Superliga player ever.

Later, his cross hit an opposing defender's arm in the penalty area, but his teammate Riccardo Orsolini missed the attempt from 12 yards.

However, Kristiansen's shot on target in the 89th minute resulted in a blunder by goalkeeper Boris Radunović, and Giovanni Fabbian scored from the rebound.

After being named in previous squads, Kristiansen finally made his debut for the senior Danish side when he came on as a second half substitute away to Slovenia in a UEFA Euro 2024 qualifier.