After a stint at the Massachusetts Institute of Technology during World War II, Exter joined the Board of Governors of the Federal Reserve System as an economist. In 1948 he served first as adviser to the Secretary of Finance of the Philippines and then to the Minister of Finance of Ceylon (now Sri Lanka) on the establishment of central banks.
Exter left the New York Fed in 1959 to join First National City Bank (then the world’s second largest bank) as a vice president. The next year he was promoted to senior vice president. As an international monetary adviser for the bank’s International Banking Group he had special responsibilities for relations with foreign central banks and governments. In 1972 he took early retirement to become a private consultant.
Exter is known for creating Exter's Pyramid (also known as Exter's Golden Pyramid and Exter's Inverted Pyramid) for visualizing the organization of asset classes in terms of risk and size. In Exter's scheme, gold forms the small base of most reliable value, and asset classes on progressively higher levels are more risky. The larger size of asset classes at higher levels is representative of the higher total worldwide notionalvalue of those assets. While Exter's original pyramid placed Third Worlddebt at the top, today derivatives hold this dubious honor.