In addition to being a member of the National Negro Committee, the precursor to the NAACP, Pillsbury was a member of the Boston Committee to Advance the Cause of the Negro, which in 1911 became a branch of the NAACP.

In 1913, he resigned his membership in the American Bar Association when that organization rejected the membership of William H. Lewis, a black assistant U.S. attorney and supporter of Booker T. Washington.

It was there he delivered his speech illuminating, defending and praising President Lincoln's role in ending slavery that became a small book, Lincoln and Slavery.

[6] In 1916 the Massachusetts legislature and electorate approved a calling of a Constitutional Convention.

[7] In May 1917 Pillsbury was elected to serve as a member of the Massachusetts Constitutional Convention of 1917, representing the Ninth Norfolk District of the Massachusetts House of Representatives.