Born in Durham, Maine, Miller attended Bates College and the School of Architecture at the Massachusetts Institute of Technology (1891–1892).
The firm designed schools, libraries, hotels and churches, as well as private residences throughout the state.
In 1926, Lester I. Beal (1887–1966) became a partner, and this firm lasted until shortly before Miller's death.
The Maine Historical Society maintains a collection of drawings by William R. Miller and successor firms.
In addition, the Maine Historic Preservation Commission has plans and elevations for the Somerset Railroad Station in Madison.