After leaving Harvard, he settled in Buffalo, New York, studied law, and was admitted to the bar in 1854.

In 1861, he joined the Union Army as an aide-de-camp with the rank of major and served on the staff of General John C. Frémont, but at the close of the Missouri campaign Dorsheimer returned to civil life, and published a series of articles in the Atlantic Monthly entitled "Frémont's Hundred Days in Missouri."

[3] During this time, he helped implement the measures against the Canal Ring, and was a delegate to the 1876 Democratic National Convention.

In 1884, he published a biography of Grover Cleveland,[5] the Democratic candidate for the presidency, and in July 1885, was appointed U.S. Attorney for the Southern District of New York.

[10] He is also chiefly responsible for bringing landscape architect Frederick Law Olmsted to Buffalo to design its park system.