From the very beginning of his career, Thomas P. Gallagher sought opportunity to have tangible impact in urban communities. In 1968, he was studying in Britain for a Ph.D. in English Literature, with aspirations of becoming a college professor, when the assassinations of Martin Luther King, Jr. and Robert Kennedy inspired him to return to the States and forge a new path of service and impact. His first job was as business manager for the Young Great Society, a community development organization that was led by a former gang leader in West Philadelphia. Among other programs, the Young Greats developed housing under federal programs for lower-income people—a catalyst that would shape his later professional and philanthropic endeavors. Gallagher came to Washington in 1972 as deputy director of a task force studying federal housing programs for then-Secretary of the Department of Housing and Urban Development George Romney. This assignment led to a nine-year tenure at consultancy Touche Ross (now Deloitte), where he became a partner with specialties in health care and real estate.

In 1982, he and a colleague founded Edmondson & Gallagher (now E&G Group), a real estate developer and management company specializing in affordable rental housing. E&G Group has renovated close to 10,000 units and currently has a portfolio of about 4,000 apartment units in the Washington, D.C. region. The company works with social service agencies and provides housing to a variety of special needs populations, including formerly homeless military veterans. He continues to guide the company as President.

Tom is very active in the cultural and business life of the Washington region.  He is an active member of the Apartment and Office Building Association (AOBA).  He is also a member of the DC Police Foundation and serves on the Executive Committee of the Federal City Council.

Gallagher has served on the Boards of The Washington Ballet (1985–2000), the Cathedral Choral Society (2002–2017), and Washington Performing Arts (2015–present). Gallagher served as board President for both the Ballet and CCS; at Washington Performing Arts, he is currently Chairman of the Board, having previously served as Chair of the Development Committee and a member of the Finance Committee

Concurrently, Gallagher also runs his own nonprofit organization, Turnaround, Inc., whose mission is to provide job training and long-term employment to people at risk of homelessness. In addition to the performing arts and Turnaround, Inc., Gallagher supports the CAIR Coalition, which provides free legal help to undocumented people at risk of deportation, and the Dorobo Fund, which preserves the livelihood of hunter-gatherer and pastoral societies in northern Tanzania.