Between 1945 and 1950, Mortensen scored in twelve consecutive rounds of the FA Cup, including in defeat in the 1948 FA Cup Final. He also scored in 15 consecutive First Division games in 1950–51, although he missed two games due to injury during the span. Mortensen topped the First Division goalscoring charts in 1950–51, with 30 goals. His most famous performance occurred two years later in the 1953 FA Cup Final, when he helped Blackpool to a 4–3 win against Bolton Wanderers, after being 3–1 down, by scoring a hat-trick. Mortensen's third goal came with just a minute left in the game, and Bill Perry's injury-time goal sealed the victory.