Hesslén was born in Visseltofta, Scania, Sweden, to Måns Sunesson, a farmer and district judge (häradsdomare) and Sara Månsdotter. Hesslén's father hired a Norwegian student in the area to teach his son privately. His studies continued in Lund in 1745, where he received his magister degree in 1751. In 1755 he became docent in theology and in 1760 an adjunct professor. Hesslén was ordained in 1767. Awarded a doctorate in theology in 1769, he returned to Lund University in 1775 as the fourth professor of theology. He became first professor of theology and dean of Lund in 1794.[2][3]
In 1777 and 1794 Hesslén was on the lists of nominees for the office of Bishop of Lund without success. After the death of Bishop Petrus Munck in July 1803, he received the majority of the votes in the election and was appointed – because of the absence of King Gustav IV Adolf due to the war – to the episcopate only in March 1805. He was installed by Archbishop Jacob Axelsson Lindblom in the summer of 1805 and served as bishop of the Diocese of Lund until his death, without making any particular impact there. His special commitment continued to be to the university, of which he was pro-chancellor.
Hesslén did not publish any writings. He became renowned for his good financial standing and amassed a considerable fortune; he was also known for his satirical attacks on those who opposed him.[2] He was described by poet Esaias Tegnér as a "Christianized Socrates".[3]
^ abcMellin, Gustaf Henrik (1847). "152. Nils Hesslén". 427 porträtter af namnkunniga svenske män och fruntimmer (in Swedish). Stockholm: P. G. Berg. p. 79. Archived from the original on 2023-03-19. Retrieved 2023-03-19 – via Project Runeberg.
^ abKirke-leksikon for Norden (in Danish). Vol. 2: F–K. Jydsk forlags-forretning. 1904. p. 421.
^Sjöström, Carl (1897). "Inspektorer. Biktfäder.". Skånska Nationen före afdelningarnes tid (1682–1832) (in Swedish). Lund: E. Malmströms boktryckeri. p. 31. Archived from the original on 2023-03-19. Retrieved 2023-03-19 – via Project Runeberg.
Palmblad, Vilhelm Fredrik; Werner, Karl Fredrik (1840). "Hesslén, Nils". Biographiskt lexicon öfver namnkunnige svenska män (in Swedish). Vol. 6. Hartman-Järta. Uppsala. pp. 130–133 – via Project Runeberg.