[1] He is the younger brother of Danish football player Andreas Ulrich Mortensen.

Casper U. Mortensen changed scenery as he joined Fredericia HK in the 2009–2010 season, on a two-year contract.

This made him number 2 on the top scorer list, making him the league player with the highest scoring average pr match, as he played 6 matches less than other league player due to injuries.

[5] After playing the European Championships in Poland with the national team of Denmark in 2016, Mortensen signed a contract with TSV Hannover-Burgdorff for the rest of 2015–2016 season.

The season 2017-2018 went on to be one of the bests in Mortensen career, where he finished the season strong with Hannover-Burgdorf (6th place in the league - best result in history of Hannover) and was crowned with a personal trophy - the topscorer in the Bundesliga with 230 goals in total.