After graduation, Robert M. Nevin moved to Dayton, Ohio, where he pursued the study of law under the tutelage of Thomas O. Lowe. When Lowe was elected to the bench of the superior court, Nevin entered the office of Conover & Craighead, well-known attorneys, where he completed his study of law until admitted to the bar in Montgomery County, Ohio in 1871.
In November 1871, Robert Murphy Nevin was united in marriage to Miss Emma Reasoner of Dresden, Ohio. They had four children.
In 1876, Robert Nevin entered into professional partnership with Alvin W. Kumler, and the firm of Nevin & Kumler was maintained until the election of Kumler as judge of the Montgomery County Court of Common Pleas, by which time it was the oldest continuous law partnership in south central Ohio. R. M. Nevin was a distinguished criminal lawyer and occupied a prominent place at the Ohio bar as counsel for the New York Central Railroad for thirty years (1882–1912).
Robert M. Nevin was a lifelong Republican, very active in state and local politics, and served as chairman of the Republican county committee of Montgomery County. He was a delegate to 14 Republican state conventions in 15 years, his name being put in nomination for Secretary of State and Governor at different times. In 1893, he nominated William McKinley for Governor of Ohio at the state convention in Columbus, Ohio. Nevin was a delegate to the Republican National Convention in 1892.
History of the Republican Party in Ohio. Chicago: Lewis Pub. Co., 1898, 1579 pgs.
Gilkey, Elliot Howard, The Ohio Hundred Year Book. Columbus: F.J. Heer, state printer, 1901, 779 pgs.
Conover, Frank, editor. Centennial Portrait and Biographical Record of the City of Dayton and of Montgomery County, Ohio Logansport, IN: A. W. Bowen & Co. (Press of Wilson, Humphreys & Co.), 1897, 1310 pages.