Masci has been on the faculty of the Icahn School of Medicine at Mount Sinai since 1982, Since 2003, he has been a Professor of Medicine, infectious disease. From 1999 to 2002, he was the director of second year curriculum at Mount Sinai. He has served as a visiting professor at the Pavlov University Medical School in St. Petersburg, Russia from 2009 to 2012. He has also served as project coordinator and technical lead on HIV-focused clinical partnerships in Ethiopia and Russia between 2004 and 2014 .