[1] Mayer's research on inflammatory bowel disease pathogenesis was among the first to demonstrate the role played by T-regulatory cells in setting the stage for chronic mucosal inflammation (IBD).

He completed an internship and a residency in Internal Medicine at Bellevue Hospital Center/New York University in 1979 and a fellowship in Gastroenterology at the Mount Sinai Medical Center in 1981.

[6] He was Associate Professor of Medicine and Microbiology at Mount Sinai Medical Center from 1985–1986 and was named Director of the Division of Clinical Immunology in 1986.

From 1992–1997, Mayer served on the Immunological Sciences Study Section of the National Institutes of Health.

He has served as Chairman of the National Scientific Advisory Committee of the Crohn's & Colitis Foundation of America since 2008.