Born in Brantford, Ontario, Hillier was educated at the University of Toronto, where he gained successively his BA (1937), MA (1938), and PhD in physics (1941). He went to live and work in America in 1940 and became a naturalized citizen in 1945. From 1940 to 1953 he worked for the Radio Corporation of America (RCA) Laboratories as a research physicist, primarily on the development of the electron microscope.

Many efforts were being made around the world to develop a commercial electron microscope that could offer higher resolution than optical microscopes. It had been known since the 1920s that a shaped magnetic field could act as a ‘lens’ for electrons, and in the 1930s the first electron micrographs had been taken. Hillier and his colleagues at RCA designed and built the first successful high-resolution electron microscope in America in 1940; they had in fact been anticipated by Ernst Ruska and Max Knoll who had produced a similar machine for the Siemens and Halske Company in Germany in 1938. The outbreak of World War II prevented commercial development and exploitation of the German machine.

Hillier made many instrumental advances to the electron microscope. By 1946, he had achieved resolutions (magnifications) approaching close to the theoretical limits. He also involved himself in the development of techniques for the preparation of viral and bacteriological samples for examination.

Hillier's career at RCA continued with only a short break to his position of executive vice-president for research engineering. He was principally concerned with research management, and served on various American governmental, research, and engineering committees.