After studying law in the office of a Dayton lawyer for a year, Austin began teaching mathematics at Milford High School in September 1904. He married Lena Lee Morey of Battle Creek, Michigan, on December 22, 1910, and the couple had two children.
In 1912 Austin accepted an appointment as head of the mathematics department at Oak Park and River Forest Township High School in Illinois. He remained in this position until his retirement in 1944. In 1919 Austin was granted a M. A. degree from the University of Chicago. He co-authored textbooks on plane and solid geometry in the early 1930's.
In 1913 Austin and several other mathematics teachers and professors from the greater Chicago area organized the Chicago Men's Mathematics Club. Austin was immediately elected to a three-year term as the club's first president. That organization is still in existence today as the Metropolitan Mathematics Club of Chicago. Its membership lists over the years read like a "Who's Who" of prominent mathematicians and mathematics educators from the Midwest.
Between 1912 and 1919 Austin grew increasingly disturbed by the destructive criticism being aimed at mathematics teaching in the schools. He was frustrated because there was no national organization to represent the mathematics teachers or to suggest ways of improving the teaching of mathematics. Correspondence with mathematics teacher leaders in other states led to the organization of the National Council of Teachers of Mathematics during a meeting of the National Education Association at Cleveland, Ohio, on February 24, 1920.
On that date 127 enthusiastic teachers of mathematics, representing 20 states and more than a dozen local associations, heard addresses by Austin, Marie Gugle of Columbus, C. N. Moore of the University of Cincinnati, Ohio native J. W. Young of Dartmouth College, and others. Austin was elected as the first NCTM president. He served in this office for one year and later served a term as NCTM vice president (1927-29) and four terms on the board of directors.
Austin was also influential in making The Mathematics Teacher the official NCTM journal. It had been established in 1908 as a quarterly publication of the Association of Mathematics Teachers of the Middle States and Maryland, under the editorship of William H. Metzler of Syracuse University. Following negotiations between Austin and Professor Metzler in the summer of 1920, the first issue to be published by the Council appeared in January 1921.
The first NCTM Yearbook, A General Survey of Progress in the Last 25 Years, was published in 1926, under the auspices of a committee chaired by C. M. Austin. Among its featured articles is a republication of E. H. Moore's 1902 presidential address to the American Mathematical Society, "On the Foundations of Mathematics."
Article by David E. Kullman