He graduated in medicine from the University of Warsaw in 1890, and studied further in Berlin. From 1892-1912 he worked as a surgeon at the Infant Jesus Hospital in Warsaw, specialising in urology. Despite this, his most notable reports were on several cases of inflammatory bowel disease. On May 10, 1903, Medycyna, a weekly medical newspaper, published an article in which he described several cases of intestinal disease, concluding in at least one case: "we suspected a chronic inflammatory process in the wall of the gut."  He wrote three further articles describing cases for the Pamiętnik Towarzystwa Lekarskiego Warszawskiego (Annals of the Warsaw Medical Association) between 1903–1905, consistent with what is now known as Crohn's disease, although the evidence is not conclusive.  In one of these articles, in 1904, he reported a meeting of the Warsaw Medical Society, at which he presented a surgical specimen of an inflammatory tumour of the terminal ileum with a fistula to the ascending colon. 
The disease was described again in 1932 by US American gastroenterologists Burrill Bernard Crohn, Leon Ginzburg and Gordon D. Oppenheimer, and since that time, due to the precedence of the name Crohn in the alphabet, it has been known in the worldwide literature as Crohn’s disease. Only in Poland is it known as Leśniowski-Crohn’s disease.
During 1912-1914, he was director of Warsaw’s St Anthony Hospital, then until 1919, was the head of the surgery department in the Holy Ghost Hospital in Warsaw. During 1919-1936, he was professor of surgery at the University of Warsaw. He also wrote a textbook of general surgery.
- Lichtarowicz, A.M.; Mayberry, J.F. (August 1, 1988). "Antoni Lésniowski and his contribution to regional enteritis (Crohn's disease)". Journal of the Royal Society of Medicine 81 (8): 468–470. PMC 1291720. PMID 3047387. Retrieved 2008-03-28.
- Kornel Gibiński (April 1, 1989). "[Re:] Antoni Leśniowski and his contribution to regional enteritis (Crohn's disease)". Journal of the Royal Society of Medicine 82 (4): 243–4. PMC 1292101. PMID 2716027. Retrieved 2008-03-28.
- W. Bartnik (December 2003). "Inflammatory bowel disease - Polish contribution". Journal of Physiology and Pharmacology (Kraków: Polish Physiological Society) 54 (S3): 205–210. PMID 15075474. Retrieved 2008-03-28.