Boutwell during the 1880s and 1890s practiced international and patent law from offices in Boston and Washington, D.C. His business included working for the United States and other national governments as counsel to several bilateral diplomatic commissions. In the first, running from 1880 to 1884, he represented the US in regard to claims involving France which mostly emanated from the Civil War. He next served as counsel for Haiti (1885), and then again for the US on a commission with Chile (1893–94), which addressed claims against both governments most of whose origins were in either the War of the Pacific or the Chilean Civil War of 1891. In 1881, Boutwell turned down the appointment of Secretary of the Treasury from President Chester A. Arthur. He served for a time as a legal representative for the Kingdom of Hawaii, whose acquisition by the US he opposed.