In Johnstown, he received a limited formal education, but went on to teach school and study law although he never practiced.

Prosser returned to Pennsylvania in 1861 upon the outbreak of the Civil War to enter the Union Army.

In 1879, Prosser was appointed by President Rutherford B. Hayes as special agent of the United States Department of the Interior for Oregon, Washington, and Idaho.

He married Flora Louise Thornton in Seattle on April 6, 1880, and settled in the Yakima River valley area in 1882, where he founded the town of Prosser, Washington.

He also served as chairman of the State harbor line commission, mayor of North Yakima (1890–1903), and city treasurer of Seattle 1908–1910.