[1] Tinkham returned to public office, serving as a member of the Boston Board of Aldermen from 1900 to 1902.

In 1910 he returned to public service, being elected as a member of the Massachusetts State Senate, where he served from 1910 to 1912.

[1][2] During World War I, he served in the military;[1] Tinkham would later tell Life magazine that while touring the front as a Congressman he fired the first American shot against the Austrians.

[1] During that time Tinkham was nicknamed "the conscience of the House" for his efforts to protect voting rights for African Americans,[2] in part by highlighting of the South's disproportionate representation in the House related to that region's voting population.

[4] In 1937, a California newspaper reported "Representative Tinkham of Massachusetts, on the other hand, is emphatic in the view that we are heading for an alliance with England, France and Russia against Germany, Italy and Japan and he favors playing a lone hand and attending strictly to our own business".