Dr. Freeman graduated from Harvard College and received his MD at the University of California, San Francisco in 1979. He served as an intern, resident and endocrinology fellow in the Department of Medicine at Massachusetts General Hospital and was Chief Resident in Medicine at Mass General in 1985.

Following clinical training, he worked as a post-doctoral research fellow in the Biology Department of the Massachusetts Institute of Technology before returning to Mass General to head the Cardiovascular Health Center. He became Chief of the Lipid Metabolism Unit at Mass General in 1992.

Dr. Mason's research work has centered on the role of macrophages in atherosclerosis with particular interest in the trafficking of lipids through these cells. While at MIT, he and his colleagues cloned the first macrophage scavenger receptor. Over the years, his lab has studied the mechanisms of atherosclerotic foam cell formation and the impact of lipids on activation of vascular inflammatory responses. The lab has also made fundamental contributions to our understanding of reverse cholesterol transport via its work on the topology and biochemistry of the ABCA1 transporter, mutations in which cause Tangier disease.

In the last two years, the lab has expanded its interest to the other ABCA transporters, all of which appear to play a role in cellular lipid trafficking. Mason remains clinically active, teaching medical students, residents, and endocrine fellows rotating through Mass General clinical services and he continues to direct the Mass General Lipid Clinic that he founded in 1986.