Johnson was recruited by the MIT Radiation Laboratory to work on World War II related radar research.
After the war Johnson began graduate studies in astronomy at University of California, Berkeley where he completed his thesis under Harold Weaver in 1948.
In the following years working at Lowell Observatory, University of Wisconsin–Madison, Yerkes Observatory (where he met William Wilson Morgan), McDonald Observatory, University of Texas–Austin, the Lunar and Planetary Laboratory in Tucson, Arizona, and the National Autonomous University of Mexico he applied his instrumental and electronic talents to developing and calibrating astronomical photoelectric detectors.
Johnson was awarded the Helen B. Warner Prize by the American Astronomical Society in 1956.
He is remembered for introducing the UBV photometric system (also called the Johnson or Johnson-Morgan system), along with William Wilson Morgan in 1953,[1] which he later expanded also with infrared filters in the 1960s.