He was a member of the London Revolution Society and in February 1792 he offered to help Talleyrand secure British neutrality in the European wars involving France.
He and his wife moved to Paris in April to open a sal ammoniac factory and established himself as part of the British expatriate community, including Helen Maria Williams with whom he had a subsequent long love affair.
Stone, while saddened by the September Massacres, believed they were necessary for the revolutionary to succeed and he and other expatriates celebrated French military victories.
During the reign of terror, Stone paid 12,000 francs to help Stéphanie de Genlis's husband escape from prison, but she later refused to pay this debt back.
Throughout these troubled years, Stone continued to write openly to his brother William, who was tried for treason (unsuccessfully) in January 1796 on the basis of the letters.