Dennis Matthew Riordan was born in 1848 in
Troy,
New York. Dennis was the son of Irish immigrants
Timothy Riordan and
Elicia Knowles. Dennis had one younger sister, Nell. The Riordans brought the strong Irish tradition of a hard work and family unity with them. Like many
immigrants, Dennis had little formal education. By the time he was 10 years old he had all the schooling he would ever have. Dennis worked many different jobs to help his family while in
Chicago, but when he was 16 he enlisted in the
Union Army. He fought in the
Civil War and fortunately was never injured. He remained in Chicago for three years after the war, but in late 1868 Dennis moved to
Nevada to pursue his interest in
mining. He worked there for the next twelve years. During his time in Nevada Dennis learned the business of mining and also learned about the opportunities of logging. In 1880 Dennis moved to
Arizona and was involved of the establishment of copper mines in
Tucson and
Bisbee. Both mines were only marginally successful, and soon Dennis was looking for more work.
He moved to what is now Flagstaff and became a supervisor at Ayer Lumber Mill on May 10, 1884. Just three years later he purchased the operation and renamed it. Arizona Lumber Company was founded on June 20, 1887 with Dennis Riordan as general manager. He persuaded his two half brothers, Timothy and Michael to join him in Flagstaff. Dennis was more than a businessman, he was also very active in the community. He was active in the library association and was one of the first members of the Flagstaff Board of Trade. He even was instrumental in the separation of Coconino County from Yavapi County and is said to have named Coconino County.
In 1897 Dennis sold his stake in Arizona Lumber Company to his two brothers and F.W. Sisson. His reasoning behind this move is unclear, but it is probably due to his nagging wanderlust. He traveled, mined, and became head of mining operations for General Electric. He died in San Francisco in 1928.