He was raised near New York City and received his undergraduate education at Princeton University.
Some of Gallagher's major scientific contributions include a review article he wrote along with Sandra Faber in 1979[2] that convinced most astronomers that dark matter was real[citation needed].
In the 1980s, with Deidre Hunter, he made the first substantial studies of dwarf irregular galaxies.
He was later led research on some of the first methods for quantifying star formation in galaxies.
Gallagher was one of the leaders of the team which built the Wide Field Planetary Camera 2 for the Hubble Space Telescope.