Thornton established himself as an architect in Olneyville (then part of Johnston) in the 1890s. He remained there until about 1900, when he formed a partnership with Richard C. Sanders, another young architect, in central Providence. Sanders & Thornton was dissolved in 1902, and Thornton partnered with his cousin, Henry C. Thornton. They split about 1903, when Henry left to partner with his brother-in-law, Edwin T. Banning. However, Thornton continued as Thornton & Thornton until his death in 1911.
Private practice, before 1899:
- 1895 - Concord Street Grammar School, Roosevelt St. & Sterling Ave., Providence, Rhode Island. Then in Johnston. Demolished.
- 1899 - St. Alban's Episcopal Church, 1964 Smith St., Centerdale, Rhode Island. Altered.
Sanders & Thornton, 1900-1902:
- 1900 - Webster Avenue Primary School, 241 Webster Ave., Providence, Rhode Island.
- 1901 - St. Paul's Episcopal Church, 50 Park Pl., Pawtucket, Rhode Island.
- 1902 - Emma A. Butts House, 329 Olney St., Providence, Rhode Island.
- 1902 - Douglas Avenue Fire Station, 137 Douglas Ave., Providence, Rhode Island.
Thornton & Thornton, 1902-1911:
- 1903 - Main Building, Rhode Island State Sanitarium, Pascoag, Rhode Island. Demolished.
- 1903 - Rhode Island Building, Louisiana Purchase Exposition, St. Louis, Missouri. Demolished.
- 1904 - Chester High School, Middlesex Tpk., Chester, Connecticut. Demolished.
- 1906 - Harold A. MacKinney House, 215 Arlington Ave., Providence, Rhode Island.
- 1907 - Churchill House (Rhode Island Women's Club), 155 Angell St., Providence, Rhode Island.
- 1910 - E. T. Davis Block, 388-392 Weybosset St., Providence, Rhode Island. Demolished.
- Annual Report of the State Board of Education, January, 1896. 1896.
- Engineering News 18 Dec. 1902: 218.
- Angell, Frank C. Annals of Centerdale. 1909.
- Engineering News 20 Sept. 1900: 91.)
- American Architect and Building News 22 Dec. 1900: xi.
- Olney Street-Alumni Avenue Historic District NRHP Nomination. 1989.
- Woodward, Wm. McKenzie. Providence: A Citywide Survey of Historic Resources. 1986.
- American Architect and Building News 11 July 1903: xii.
- School Board Journal Aug. 1904: 25.
- "Providence Preservation Society Records for 215 Arlington Avenue". http://gowdey.ppsri.org/. n.d. Web.
- Mitchell, Martha. Encyclopedia Brunoniana. 1993.
- American Contractor 16 July 1910: 51.