He earned his Bachelor of Science degree at the University of California and later studied in Paris and Berlin.
In 1909, Seares joined the Mount Wilson Observatory, where he remained for 36 years, 15 of them as assistant director.
Seares was elected to the American Philosophical Society in 1917 and the United States National Academy of Sciences in 1919.
Seares used astrophotography as part of Jacobus Kapteyn's effort to uncover the structure of the sidereal universe through research of "selected areas."
Seares also made contributions to the measurement and interpretation of stellar color indices and wrote on the brightness of the Milky Way compared to spiral nebulae, which were hypothesized (but not yet fully established) to be other galaxies.