Placed at the head of the "Société vinicole de la Gironde" (future Domaine Clarence Dillon SA), was Seymour Weller the nephew of Anne Dillon, Clarence's wife. He took charge of the modernization of the estate and oversaw his uncle's plans for the redecoration of the château. He installed electricity, new plumbing,tidied the park and grounds, trimmed the trees, cleaned up the chais, and updated the equipment.

To oversee the property, he retained Georges Delmas as manager. He also kept many of the same families to work in the vineyards. Some had been there since the time of Larrieu.

At the start of the war in 1939, following the wishes of his uncle, Seymour Weller arranged for Château Haut-Brion to be set up as a hospital and offered to the French government. Much of the furniture was parceled out for storage with employees of the estate. He also took over the lease of his uncle's flat in Paris and used it throughout the occupation as a listening post of the BBC. He met and married his French wife, Thérèse, at this time. She was working for Granvins, the Bordeaux brokerage house owned by Haut-Brion.

Seymour Weller was an avid francophile. He spoke perfect French but with a strong American accent. Naturalized as a French citizen in 1939, he was soon elected mayor of Neufles-Saint-Martin in the Eure, his weekend home outside of Paris. He remained Mayor for fifteen years. When finally he decided to give up his mandate they moved to a home that they bought in Amboise, half way between Paris and Haut-Brion.

In the late sixties Seymour Weller, after a bout of thrombosis, underwent the amputation of one of his legs. This in no way discouraged him. He insisted upon learning to walk with an artificial limb and returned to work. Finally in 1975, at the age of 83, Seymour Weller retired as President of the company. His cousin's daughter, Joan Dillon, then Princesse Charles de Luxembourg, replaced him. Seymour Weller remained active as a member of the Board at Haut-Brion until he died in 1979, one week after the death of his uncle, Clarence Dillon.