His father, John Hennessy Saul, was a horticulturist and landscape architect, born in County Cork, Ireland and emigrating to the United States in 1851 to take over planning and development of the National Mall in Washington D.C.[1][2] His mother, Rosina Mary Lawley, was born in Bath, Somerset, England[3] and his parents had married in a Catholic ceremony in Bath in 1850.

In 1891, he handled the sale of his father's botanical nurseries, taking accepting promissory notes from buyers which he resold to the public and in doing so, he formed Washington D.C.'s first significant mortgage bank, the B. F. Saul Company.

[1] In 1899, Saul founded a retail bank, the Home Savings Bank, which provided financing to several of Washington D.C.'s prominent businessmen including Malcolm Gibbs, the founder of Peoples Drug, and real estate developer Harry Wardman.

[2] As payment, Saul received a large block of American Security and Trust stock and became chairman of its executive committee.

This diversification soon proved beneficial and is the primary reason his bank survived the Stock market crash of 1929 and the Great Depression[1] becoming, at the time, the largest financial services company in Washington D.C.[1][5] In 1898, he married Frances E. Maguire in Manhattan, New York.