Katharine Margaret Alice Macmillan, Viscountess Macmillan of Ovenden (née Ormsby-Gore; 4 January 1921 – 22 January 2017), was the daughter of the 4th Baron Harlech, granddaughter of the 4th Marquess of Salisbury, great-granddaughter of the 10th Marquess of Huntly and daughter-in-law of Prime Minister Harold Macmillan, 1st Earl of Stockton. On 22 August 1942, she married Conservative politician Maurice Macmillan, making her the daughter-in-law of fellow Conservative politician Harold Macmillan, who would serve as prime minister from 1957 to 1963. They had five children together: She was Vice-Chairman of the Conservative Party in 1968 and was created a Dame Commander of the Order of the British Empire (DBE) in 1974. She became Viscountess Macmillan of Ovenden in February 1984 when her father-in-law, former British prime minister Harold Macmillan, accepted a peerage and was created Earl of Stockton. Her husband died the following month, so never inherited the earldom from his father. She died on 22 January 2017.