Gallagher grew up in the ski resort of Mammoth Lakes, California, where he was the quarterback for the Mammoth Huskie football team before he graduated from high school in 1979. Gallagher was graduated from the University of San Diego where he majored in business and played quarterback for the Torrero football team.
Based in Los Angeles, Gallagher was co-anchor with Martha MacCallum of The Live Desk, a general assignment correspondent and substitute anchor for Studio B and Fox Report. During his time at Fox, Gallagher has reported on the Space Shuttle Columbia disaster and 2004 Southeast Asian tsunami.
He has also worked at Fox's Chicago, San Francisco, and New York City bureaus. Previously Gallagher worked at WCPX (now WKMG) in Orlando, Florida, KYEL (now KSWT) in Arizona, KVBC (now KSNV) in Las Vegas, Nevada and KTVB in Boise, Idaho.