Robert Closson Spencer Jr. was born in Milwaukee, Wisconsin on April 13, 1864. He graduated from the University of Wisconsin with a degree in mechanical engineering in 1886 and then entered the architecture program at the Massachusetts Institute of Technology, one year after fellow Prairie School colleague Dwight Heald Perkins. Spencer left M.I.T. in 1889 to work in the Boston architectural firm of Wheelwright & Haven and then Shepley, Rutan & Coolidge. He married Ernestine Elliott on November 28, 1889 and in 1891 won the Rotch Traveling Scholarship which provided him the opportunity to travel throughout Europe with his wife for the following two years studying architecture.

The Spencer family returned to the United States in 1893 and Robert went back to work for Shepley, Rutan & Coolidge, but this time in their Chicago office. He stayed with the firm until 1895 when he established his own practice in Chicago next to Frank Lloyd Wright in the Schiller building.

The following year he moved his office into the loft of the Steinway Piano building and shared offices with Dwight Perkins and Frank Lloyd Wright. He was known as a very prolific writer and in 1900 wrote the first of many articles for the Architectural Review, highlighting the work of his friend Frank Lloyd Wright. In the same year he authored a series of articles in The Ladies Home Journal about midwestern farm houses.

In 1905 Spencer went into partnership with Horace S. Powers, a Chicago native and graduate of the Armour Institute of Technology (now the Illinois Institute of Technology). Spencer functioned as the designer and Powers contributed as the office manager. They designed residential structures primarily in Chicago and the surrounding suburbs with some additional commissions scattered across the Midwest. The partnership left time for Spencer to found The  Casement Hardware Company of Chicago in 1906. The company invented and manufactured hardware for casement windows. Their partnership lasted until 1923 when Spencer returned to private practice.

In 1928 Spencer joined the faculty of the school of architecture at Oklahoma A&M. This was short-lived as he left there in 1930 to join the faculty at the University of Florida until 1934. He then painted murals for the United States government until his retirement to Tuscon, Arizona in 1938. He died on September 9, 1953.