Osborne served two terms on the Auburn Board of Education and in 1903 was elected mayor of Auburn, serving one term. In 1913 he became chairman of the New York State Commission on Prison Reform, a post that inspired him to spend a week in Auburn Prison as “Tom Brown” to learn about conditions firsthand. Convinced thereby that (in the British statesman W.E. Gladstone's phrase) “it is liberty alone that fits men for liberty,” he founded a Mutual Welfare League of the Auburn prisoners, which assumed such responsibilities as forming committees to judge violators of prison rules and to plan entertainment events.

Osborne was warden of Sing Sing State Prison (now Ossining Correctional Facility), Ossining, N.Y., from 1914 to 1916 and, from 1917 to 1920, commander of the Portsmouth Naval Prison in New Hampshire; he instituted a Mutual Welfare League in both places. His administration at Sing Sing was a stormy one, culminating in an indictment by the Westchester County Grand Jury for perjury and neglect of duty. The charges, however, were dismissed. Osborne wrote many books on penology and founded a national society for dissemination of penal information.