Theo Keller was born on 3 August 1901 in Schongau (LU).
He attended grammar school in Engelberg (OW) and obtained his Matura in 1921. Keller then studied economics in Zurich, Freiburg, and Berlin. In 1925 he completed his doctorate in Zurich with a thesis on the power supply of the city of Zurich.
After completing his doctorate, Keller headed the statistical office of Bank Leu in Zurich for eight years. From 1934 he taught economics at HSG. He was also secretary of the Association for Sound Currency - an organisation that represented the ideas and interests of the National Bank.
After Theo Keller completed his habilitation in 1936 on the topic of bank-industry relations entitled Das Verhältnis der Banken zur Industrie (The relationship of banks to industry), he was appointed Full Professor of Economics and Finance at HSG in 1937.
Keller was elected to be president of HSG in 1944. He devoted a large part of his time to advising students on their studies and careers. He was organised internships, procured scholarships, and arranged employment for graduates. A motivated teacher, he knew how to inspire students for the subjects he taught.
During Theo Keller's term of service, several institutes were founded, including the Swiss Research Institute of Small Business and Entrepreneurship (IGW), the Institute for Technical Education in Textile and the Insurance Industry Seminar.
In 1969, Keller retired and was appointed Honorary Professor.
Keller published several articles in renowned domestic and foreign economic journals. His research earned him high esteem in academic circles as well as in business and politics. He contributed his knowledge, for example, to the Federal Commission for Labour Market Issues and the Commission for Business Monitoring. He also served on the Board of Trustees of the Swiss National Science Foundation.
Theo Keller founded the company Revisa Treuhand AG. In his free time, he was interested in baroque architecture.
Theo Keller died on 7 April 1980 in St.Gallen.