Dunn briefly practiced law in Indianapolis, then moved to Colorado in 1879, where he and his brothers prospected and looked after their father's mining interests.

In 1884 Dunn returned to Indianapolis and completed his first book, Massacres of the Mountains: A History of the Indian Wars of the Far West, 1815–1875, published in 1886.

Dunn's interest in history also led him to join other historians in revitalizing the Indiana Historical Society into an effective organization.

Dunn was appointed for two terms as the Indianapolis city controller, from 1904 to 1906 and from 1914 to 1916, and served two years as chief deputy to the Marion County treasurer, from 1910 to 1912.

[1] Dunn's father, a cattle trader, went to the California goldfields in 1849, returned to Indiana in 1854, and moved the family to a farm on the Ohio River before finally settling in Indianapolis in 1861.

[2] Dunn attended public schools in Indianapolis and graduated with a bachelor of science degree from Earlham College in Richmond, Indiana, in 1874.

After graduation, Dunn returned to Indianapolis and briefly practiced law, working for the firm of McDonald and Butler, before moving with his brothers to Colorado in 1879 to prospect and look after their father's silver mine interests.

[5] Dunn contributed articles to the Denver Tribune-Republican, the Leadville Chronicle, the Maysville Democrat, and the Rocky Mountain News,[6] before returning to Indianapolis in 1884.

[7] After Dunn returned permanently to Indianapolis in 1884, he resumed his law practice and completed Massacres of the Mountains: A History of the Indian Wars of the Far West, 1815–1875 using research he had begun in Colorado.

[10] Dunn further supported himself by writing political editorials for local newspapers,[11] including work for the Indianapolis Sentinel, which he also edited for a time.

[12] In 1886 Dunn joined fellow Hoosiers, including William H. English, Daniel Wait Howe, Major Jonathan W. Gordon, and other Indianapolis historians to "revitalize" the languishing Indiana Historical Society.

[17][18][19] In addition to his position as state librarian, Dunn's other jobs included two terms as the Indianapolis city controller, from 1904 to 1906 and from 1914 to 1916, and two years as chief deputy to the Marion County treasurer, Frank P. Fishback, from 1910 to 1912.

[26] Although it was a move forward, Dunn felt it fell short of eliminating vote buying entirely and continued his efforts for additional election reforms in the state.

[29] In 1914, as the city controller appointed by the Democratic mayor Joseph E. Bell, Dunn was criticized in the Indianapolis News for using interest earned on contractors' guaranty bonds for personal gain.

[30] Although there was no law prohibiting this, and previous occupants of the office had also followed this custom, Bell ordered Dunn to stop the practice in December 1915.

Dunn was not prosecuted; however, "Reddington, Pugh, and John Shaughnessy, a former bookkeeper in the controller's office, were indicted by a Marion County grand jury.

[34] Following the publications of Massacres of the Mountains and Indiana: A Redemption from Slavery, Dunn continued to research and write about other state and local history topics.

While he remained a part-time historian, Dunn wrote for and edited several Indiana Historical Society publications and contributed articles to other scholarly journals.

[35] Dunn's career as a newspaper journalist, his primary source of income, gave him the opportunity to write about state and local politics.

[40] Dunn's two-volume book, Greater Indianapolis: The History, the Industries, the Institutions, and the People of a City of Homes, published In 1910, is considered to be his greatest work.

[50] While in his sixties, Dunn traveled to Hispaniola for two months in 1921, visiting Haiti and Santo Domingo to evaluate the area's mineral resources and hoping to identify profitable manganese mines for a group of American investors.

[51] Returning to the United States in early 1922, Dunn wrote about his Haitian adventures as well as his studies of the island's dialects and voodoo cult.

"[59] Two of Dunn's efforts ended unsuccessfully: the first, obtaining precious metals in the American West and Hispaniola; the second, and far more controversial, a desire to write a new Indiana state constitution, which included language that would have removed voting privileges for many immigrants and blacks.