He also worked on inscriptions from Max Mallowan's excavations at Nimrud, served as the field director of the Scandinavian Joint Expedition to Sudanese Nubia, and published a number of popular history books on Assyriology in Danish, including his magnum opus, The People of Ancient Assyria (1963).

Læssøe also worked in the United States, first on the Chicago Assyrian Dictionary (1948–1951) and then as a visiting professor at the University of California, Berkeley (1953–1955 and 1966–1967).

[3][4] In 1928, the family moved to the suburbs of Copenhagen, where his father managed a branch of the department store Magasin du Nord, and Jørgen attended a private school in Farum.

Since the curriculum required knowledge of a non-Indo-European language, he took a course in Akkadian taught by Assyriologist Otto E. Ravn, which thereafter became the main focus of his studies.

[2][3] His education was disrupted by the German occupation of Denmark, during which Læssøe was active in the Danish resistance movement.

He received his doctorate in 1955 with a thesis on the bīt rimki, an Assyrian ritual, and in 1957, succeeded Ravn as Professor Extraordinaire of Assyriology.

[8] Between 1956 and 1960, Læssøe worked as the epigrapher on Max Mallowan's excavations at Nimrud,[7][9] publishing two papers on inscriptions from the reign of Shalmaneser III.

[11][12] During his time there, he became friends with Mallowan and his wife Agatha Christie, who wrote a number of verses about Læssøe and his Danish colleagues, whose names the English team found unpronounceable.

[9] The excavations uncovered an Old Assyrian palace complex and substantial cache of cuneiform tablets,[14] which occupied Læssøe for much of the rest of his career.

[2] In an obituary, Læssøe's student Jesper Eidem highlighted his "peculiar devotion" to Assyriology: "Jørgen was a learned scholar of extraordinary intelligence and talent who insisted on the highest standard in his work, but who simultaneously refused to regard his profession as more than a schoolboy hobby in comparison with more pressing human and personal concerns.