Mark was born and raised in San Francisco. He studied critical theory, photography, and film at Harvard University, graduating in 1983.
In 1986 he moved to New York City, where he joined the AIDS Coalition to Unleash Power (ACT UP)/New York in 1988. He was especially active in ACT UP's Treatment + Data (T+D) Committee, helping to plan and execute ACT UP's "Seize Control of the FDA" demonstration in October 1988 and its "Storm the NIH" demonstration in May 1990.
Mark authored and edited numerous reports for ACT UP, including, most notably, A Glossary of AIDS Drugs, Treatments & Trials (1988); AIDS Drugs Now (1989); A Critique of the AIDS Clinical Trials Group (ACTG) (1990); Target City Hall (1990); and T+D's three National AIDS Treatment Research Agendas (1989-91). He also wrote a column on treatment activism for Outweek magazine.
Mark was a founding member of the ACTG Community Constituency Group (CCG), serving from 1990-93 on the ACTG Opportunistic Infections (OI) and Primary Infection Committees.
In 1992, along with others from T+D, Mark left ACT UP to found TAG. 1992 was a banner year for the Treatment Action Group, with Harrington and Gregg Gonsalves' ground-breaking report "AIDS Research at the NIH: A Critical Review," (zipped file) which led to provisions restructuring the NIH AIDS research effort and strengthening the NIH Office of AIDS Research (OAR) in the NIH Revitalization Act of 1993.
At the 8th International AIDS Conference in Amsterdam during July 1992, Mark delivered a plenary address entitled "Pathogenesis & Activism", using slides of his own HIV-infected lymph nodes to demonstrate the need for activist engagement with the basic science of HIV infection. In 1993, Harrington wrote "The Crisis in Clinical AIDS Research," (zipped file) an exposé of inadequate clinical trials then being carried out by the ACTG, the Department of Defense, and others.
In 1994-95, Mark edited and co-wrote two TAG reports which were critical in the development of the HIV protease inhibitors: "Rescuing Accelerated Approval: Moving Beyond the Status Quo" (1994) and "Problems with Protease Inhibitor Development Plans" (1995)(zipped file). He served as an ad hoc community representative at several meetings of the FDA Antiviral Drugs Advisory Committee. He also served on the NIH AIDS Research Program Evaluation Working Group (the Levine Committee), whose 1996 report called for sweeping restructuring of the NIH AIDS research program.
In 1997 Mark received a fellowship from the MacArthur Foundation. Later that year he became TAG Senior Policy Director. In 1998 he delivered a plenary speech on "Cure: Myth or Reality?" at the 12th World AIDS Conference in Geneva, where he showed the impact of progressive HIV infection on his own lymph node, and the impact of highly active antiretroviral therapy [HAART]; in addition, he called for studies of "when to start" HAART, and for treatment interventions useful in developing countries.
In 1999 he helped organize the TAG/Foundation for AIDS & Immune Research (FAIR)/Project Inform sponsored Structured Treatment Interruptions (STI) Workshop. He is a member of the "US Department of Health & Human Services/Henry J. Kaiser Family Foundation Panel on Clinical Practices for the Treatment of HIV Infection" and, with Dr. Charles C.J. Carpenter, the author of "Hit HIV-1 hard, but only when necessary".