Reuben Miller lll was born and raised in Pittsburgh. Miller was an iron and steel pioneer who worked to refine steel production. He was President of the Crucible Steel Company. He later became the company's Board of Directors Chairman. Miller served as treasurer of the Bessemer Steel Company, which was later sold to Andrew Carnegie, becoming the Homestead Mills.
With the outbreak of the Civil War, Miller joined the Pennsylvania Volunteers. Miller gave charitable donations to churches and other institutions. He was a member of the Chamber of Commerce.
Miller died at his home on Fifth Avenue in Pittsburgh.
His obituary stated "few men were known so widely, especially in financial and industrial circles, as Mr. Miller. To him is attributed much credit for the great financial and industrial strides that Pittsburgh has made during the past 40 years. His name, in fact, was associated with practically every big movement and development in the history of the city during that period."