Jens Jørn Mortensen began training as a weightlifter in earnest after World War II and first appeared at the Danish national championships in 1950 as a middle-heavyweight. He made his début at the European Championships that same year. His next major stop was the 1952 Helsinki Olympics, where he placed 15th among 20 entrants overall. The following year, he won a silver medal at the European Championships and came in sixth at the Worlds. He continued to compete through 1965, taking part in 13 editions of the national championships. By career he started off as a butcher, but his weightlifting commitments interfered with his job and, following the conclusion of his competitive life, he worked as a bus driver.