On 18 August 1947, in New York City, New York, Mannheimer married Charles W. Engelhard Jr. (1917–1971), vice-president of Baker & Co. Inc. and heir to Engelhard Industries, a New Jersey-based minerals conglomerate. The couple lived in Far Hills, New Jersey, where they raised golden retrievers and thoroughbred racehorses, including the fabled Triple Crown of Thoroughbred Racing champion, Nijinsky. They had numerous homes, including Cragwood, a 1920s neo-Georgian mansion in New Jersey, a country house in South Africa, and residences in London, Paris, Maine, Nantucket, New York City, and Quebec's Gaspé Peninsula.