[1] At a young age he decided to pursue architecture as a profession, and as a teenager entered the office of Tallman & Bucklin, then Providence's leading architects.

[3] The resulting firm of Hartshorn & Wilcox lasted until his death, though works designed during this time are frequently assigned to one architect or the other.

[4][1] Hartshorn joined the American Institute of Architects in 1875 as a Fellow, and was one of the founding members of the Rhode Island chapter the same year.

[1] At least one of Hartshorn's works, designed in association with Wilcox, has been listed on the United States National Register of Historic Places.

At least two Providence architects trained in Hartshorn's office: Wilmarth H. Colwell, who later formed a partnership with Thomas J. Gould,[6] and Edward L. Angell, in practice in New York City after 1883.