J. Gardner Cassatt was a financier. He married Jennie Carter of Virginia in October 1882. He was the brother of the Impressionist painter Mary Cassatt and Alexander J. Cassatt, an engineer, collector and later President of the Pennsylvania Railroad.
Whistler painted the portrait of J. G. Cassatt's sister-in-law Lois Cassatt, née Buchanan, the niece of James Buchanan, fifteenth President of the United States, in 1883, Arrangement in Black, No. 8: Portrait of Mrs Cassatt (YMSM 250). J. G. Cassatt saw the painting in September of that year and described it as a 'fine & artistic portrait' [#00546], although Whistler himself later admitted 'it is not a striking likeness' [#09014]. It was not finished and sent off until 1886/7.
Young, Andrew McLaren, Margaret F. MacDonald, Robin Spencer and Hamish Miles, The Paintings of James McNeill Whistler, New Haven and London, 1980.