After attending the Choate School in Wallingford, Connecticut, Paul Mellon studied at Yale (1925–29), receiving his BA in 1929 and developing a keen interest in English literature. Taking his mother’s advice, he then traveled to England to pursue a second degree at the University of Cambridge, where he was a student at Clare College, graduating in 1931. At Cambridge he developed a passion for horses, most especially fox hunting, which led to his first acquisitions in the field of rare British sporting books. In later years he become a leading breeder of racehorses and was a champion trail rider until well into his seventies. His finest horse, Mill Reef—considered one of the greatest horses of the twentieth century—won the Epsom Derby, Eclipse Stakes, King George VI and Queen Elizabeth Stakes, and the Prix de l’Arc de Triomphe, all in the year 1971.
After graduating from Cambridge, Paul Mellon stayed in England while his father served briefly as the US Ambassador to the Court of St. James (1932–33). On returning to Pittsburgh, he worked for a short time as a clerk at the Mellon Bank before deciding not to pursue a career in business or banking. Instead, he moved to Upperville, Virginia, with his first wife, Mary Conover Brown, whom he had married in 1935 and with whom he had two children, Timothy and Catherine. In 1941 the Mellon family moved into the Brick House, a neocolonial property designed by William Adams Delano; that same year Paul Mellon enlisted in the army, electing to join the cavalry. After two years in Fort Riley, Kansas, he served in the Office of Strategic Services in England and rose to the rank of major.
Tragically, Mary Mellon died from an asthma attack in 1946. Two years later, Paul Mellon married Rachel (Bunny) Lambert Lloyd, an eminent horticulturalist and gardener, whose fondness for French impressionist and postimpressionist painting, as well as American art, he came to share. Together they built a new house, Oak Spring, on the Virginia estate, with the Brick House becoming over time a home for the couple’s growing collection of art.