Helen Sawyer's interest in astronomy can be traced to two events: learning about the stars from her mother as a child, and an astronomy class at Mount Holyoke College. Prior to graduating in 1926, she became interested specifically in globular star clusters. When she enrolled in Radcliffe College, she began working at Harvard University with Harlow Shapley, whose interest was also with globular clusters.

Globular star clusters were of very special interest to astronomers. Cepheid variable stars located within them could be used to determine stellar distances, and hence the size of the Milky Way galaxy. Shapley had used the globular clusters to show the galaxy was larger than was commonly believed and the Sun was not at its center. A great deal of controversy arose over his hypothesis, which led to a great debate with Heber D. Curtis (1872-1942) in 1920.

While at Harvard, Sawyer met Frank Hogg, whose specialty was stellar spectrophotometry, and they were eventually married. Helen Sawyer Hogg merged her husband's specialty with her own interest and spent hours at telescope eyepieces over the years, making long time-exposure photographs of globular clusters. In this way, she discovered 142 new variable stars.

The more variables that could be found, the better astronomers could determine the structure and size of the galaxy. Summaries of hundreds of variable stars in globular clusters had been made prior to the 1930s, but no one had ever compiled a complete listing. Helen Hogg gathered all the data she could find, organized it and published the first complete catalogue of 1,116 variables in 1939. The catalogue was of immense help in documenting the known variables in the clusters. She published another catalogue in 1955 containing 329 new variables, 30 percent of which she had discovered herself.

In 1950 she was awarded the American Astronomical Society's Annie Jump Cannon award. From 1957 to 1976 she was Professor of Astronomy at the University of Toronto, and she became well-known as a popularizer of astronomy, writing a weekly astronomy column for the Toronto Daily Star for three decades. She died on January 28, 1993.

Hogg died following a heart attack on January 28, 1993, at York Central Hospital near her home in Richland Hill, Toronto, Ontario, Canada.