Cabot was born in Brookline, Massachusetts.[1] His father was Henry Bromfield Cabot, a lawyer.[2] His mother was Anne Macmaster (Codman) Cabot, daughter of Charles R. Codman.[3][self-published source] He had five siblings:[2][3] Henry Bromfield Cabot Jr. (b. 1894), Powell Mason Cabot (b. 1896), Paul Codman Cabot (b. 1898), cofounder of America's first mutual fund[4] and "Harvard's [Endowment] Midas,"[5] Anne M. Cabot (b. 1903), and Susan M. Cabot (b. 1907).
Cabot graduated from Harvard College and Harvard Law School.[6]
Cabot was president of the Boston Bar Association[9] from 1950 to 1952. He was on the board of directors of the eugenicist Pioneer Fund from 1950 until 1973,[6] and president of the Harvard Alumni Association.[7]