H. D. Mackay was a prominent Leavenworth, Kansas businessman in the 1860s and 1870s who, in partnership with Theodore C. Sears, operated the Law and Collection Office of Mackay and Sears. In 1875, Mackay served in the Kansas House of Representatives, representing the state's 19th District.
From the guide to the Letter from H. D. Mackay (Law and Collection Office of Mackay and Sears, Leavenworth, Kansas) to Lieutenant L. E. Campbell (15th U. S. Infantry, Fort Wayne, Detroit, Michigan), October 26, 1866, (University of Kansas Kenneth Spencer Research Library Kansas Collection)