In 1857, James McCann Dunn worked for a contractor delivering freight to the new Fort Leavenworth, Kansas, but in 1858, he returned to his parents' home and went into some unspecified business, until he enlisted in the Union Army, then was assigned to Company A, 97th Regiment Illinois Infantry, later transferred to Company D, 37th Regiment Illinois Infantry, and on detached service in the ambulance corps after October 16, 1864.

[2] [b] He had begun reading law in 1889 in the Garden City, Kansas office of George Lynn Miller.

In this new position, he managed the party's strategy to win the next election for the Territory's single representative to the U. S. Congress.

[1] Williams credits Judge Dunn with unifying the remaining Populists with the regular Democrats in and after the 1906 election.

In 1909, the newly constituted Oklahoma Supreme Court adopted a rotation plan for choosing a chief justice.

[1] On the second Monday of January 1911, John B. Turner was elected as Chief Justice, replacing Dunn.

Their firm was known as Dunn, White and Aiken from March 1, 1914 until it was dissolved by the surviving partners on December 31, 1938.