Sullivan graduated Phi Beta Kappa from Morehouse College in 1989 and received his Juris Doctor from Harvard Law School in 1994.
Prior to joining the Harvard Law School faculty, Sullivan served as the director of the Public Defender Service for the District of Columbia.
"[5] Sullivan also oversees the January term Trial Advocacy Workshop, an intensive three-week course for Harvard Law School students, featuring assistance from high-profile lawyers and judges from around the country.
Sullivan is also a founding fellow, along with his wife, of the Jamestown Project,[4][6] is actively involved with community organizations in the Massachusetts area, and regularly gives speeches and moderates panels on race-based violence.
[8] On June 9, 2016, it was announced, and confirmed by news outlets such as The Boston Globe, that Sullivan would be part of the head legal defense team for ex-NFL New England Patriots football star player Aaron Hernandez in a double murder case.
[10] On January 23, 2019, it was announced, and confirmed by news outlets such as CNN, that Sullivan, with Jose Baez and Duncan Levin had joined the legal defense team of Harvey Weinstein, a movie mogul facing multiple charges of rape and other sexual assault.