Her father was a diplomat of the United States, serving as the First Secretary to the American Embassy in London.

[5] In May 1908, noted American portrait painter John Singer Sargent painted a portrait of Mildred in London that was described at the time by The New York Times as "in the painter's best manner and brings out all of the innate sweetness of nature which has endeared Miss Carter to her English as much as to her American friends, all of whom agree that she has the wonderful tact and urbanity of her father.

[9] His paternal grandfather was Archibald Acheson, 3rd Earl of Gosford and his maternal grandparents were William Montagu, 7th Duke of Manchester and the former Countess Louisa von Alten.

Five years later in 1927, the couple separated and Lord Gosford went to New York City.

[14] Mildred died on September 7, 1965, at Palazzo Salis, her home in Bondo, Switzerland.