The church received support from local community leaders - the mayor of Alexandria allowed Mass to be held in his own home before St. Mary was constructed. Though the ideals of the American Revolution may have played a part in getting support, remember that Alexandria was also an international seaport. Many of the sailors would have described themselves as Catholics, and a Catholic church would help them in times of need.
Alexandria leaders were accustomed to dealing with the Catholics in Maryland. The early priests were Jesuits from Maryland, and St. Mary Church was part of the Diocese of Baltimore.