[7] He worked briefly in New York City before enrolling at Yale University and graduating from the law school in 1877.

[1][9][10] Mills moved to New Mexico Territory, where his father-in-law, Wilson Waddingham, owned significant tracts of land, and established a legal practice in 1886.

[1][8][9][10] From August 1888 till April 1890 he was partnered with Thomas B. Catron, an influential member of the territory's Republican Party.

[11] President William McKinley nominated Mills to become chief justice of the New Mexico Territorial Supreme Court with the commissioning occurring on January 31, 1898.

[8] According to Curry, Taft found the chief justice's judicial background and conservative outlook attractive and viewed the nominee as a potential governor or U.S. senator if the territory should achieve statehood.

[14] Upon taking office, Mills began lobbying efforts aimed at achieving statehood for New Mexico.