[citation needed] At the age of 17, Wellbrock competed in his first international competition as part of the German mixed 4×100 metre relay team at the 17th Luxembourg Euro Meet.

This time broke the under-20 age group record, put him in the top ten 1500 metre freestyle swimmers in that year, and qualified him for the 2016 Summer Olympics.

[3] At the 2016 Summer Olympics, Wellbrock swam in the heats of the 1500 metres freestyle, where he finished with a time of 15:23.88 which was just under 30 seconds slower than his personal best set around 3 months earlier.

[16] At the 2021 European Short Course Swimming Championships, held at the Palace of Water Sports in Kazan, Russia, Wellbrock won a gold medal in the 1500 metre freestyle on 4 November with a time of 14:09.88.

[18] At the Abu Dhabi Aquatics Festival, held in parallel with the 2021 World Short Course Championships, in December 2021, Wellbrock won a bronze medal as part of the open water 4×1500 metre mixed relay event.

[19] In his individual event, the 10 kilometre open water swim, Wellbrock won the gold medal in a time of 1:48:09.4, finishing over two seconds ahead of silver medalist Domenico Acerenza of Italy.