Gustav Julius Mortensen (born 19 March 2004) is a Danish footballer who plays as a left-back for Danish 1st Division club Vendsyssel FF, on loan from Lyngby Boldklub.
On 20 May 2021, Mortensen became the youngest debutant in the history of Lyngby - at the age of 17 years, 2 months and 1 day - when he came off the bench in a Danish Superliga game against OB on 20 May 2021.
After just five appearances (29 minutes in the league and 67 minutes in the cup), Lyngby confirmed on June 20, 2024 that Mortensen would be on loan to Danish 1st Division club Vendsyssel FF for the coming season.
[4] On July 19, 2024 he made his debut when he was in the starting lineup in the league match against FC Roskilde.
[5] On September 8, 2024, Lyngby confirmed that Mortensen - who was on loan to Vendsyssel at the time - had signed a 1-year contract extension, so his deal now expired in June 2027.