O'Brien and his family moved from Colorado to Cheyenne where he became an involved citizen, clearing the way for his years in law enforcement and politics. O'brien was appointed Deputy United States Marshal, and in 1874 he ran for sheriff of Laramie County and won.
During his years of law enforcement, O'Brien collected taxes, served a variety of legal papers, and attached and sold property by order of the Courts. He faced murderers, thieves, robbers and law breakers in his jurisdiction with courage and conviction.
Nicholas J. O'Brien was born in Wexford, Ireland, in 1839. He had come to America as a lad of nine years, in 1848. He died in 1916 in Denver, Colorado, at the age of seventy-seven years.
Information obtained from Wyoming Territorial Sheriff's, by Ann Gorzalka.