Sinkov was the son of Jewish immigrants Morris (Mordechai Eliezer) and Ethel (née Etel Constantinowsky)[1] from Alexandria, Russia, which is now Oleksandriya, Kirovohrad Oblast, Ukraine.

The small Signal Intelligence Service (SIS) organization (Sinkov and Kullback were the third and fourth employees there) had the primary mission of compiling codes and ciphers for use by the U.S. Army.

Its secondary task was to attempt to solve selected foreign codes and ciphers—this was not necessarily done for intelligence purposes but to keep the cryptanalysts abreast of new developments in the field.

William Friedman put his new employees through a rigorous course of study of his own design in cryptology, bringing them to high levels of skill in making and breaking codes and ciphers.

Friedman also encouraged other self-improvement endeavors: his employees trained summers at a camp at Ft. Meade to earn commissions in the military reserves.

This led to an unprecedented level of cooperation in communication intelligence (COMINT) between the two countries during the war, resulting in more personnel, bigger budgets, and a wider range of activities for the organization.

Sinkov and his colleagues had been shown Bletchley Park, the secret headquarters for British cryptology, and exchanged information on German and Japanese systems.

Sinkov, who demonstrated strong organizational and leadership qualities in addition to his mathematics skills, brought this group of Americans and Australians—representing also different military services from their countries—into a cohesive unit.

This SIGINT enabled consistent success in the air war against the Japanese and allowed MacArthur to win some stunning victories in the ground campaign in New Guinea and the Philippines.

In 1966, he wrote Elementary Cryptanalysis: A Mathematical Approach, one of the first books on the subject, directed at high school students and available to the general public.