Her husband has a Wikipedia article. Her father has a Wikipedia article. She does not. Her article in thepeerage is here. She was Amelia Darcy's daughter.
From Wikimedia; image resized to 27.59 cm high at 28.35 pixels/cm.
Lady Mary's dress has a criss-cross bodice, apparently armless, cinched with a waist band in this miniature portrait. She wears her dress over a chemise. This may have been painted about the time she married in 1801.
Keywords: Cosway, Mary Henrietta Juliana Pelham, Countess of Chichester, Osborne family, Pelham family, Countess, British, straight coiffure, bangs, bun, chemise, trapezoidal waistline, tight upper puffed sleeves, arm bands, bodice, vee neckline, collar, lace, strap sleeves, waist band, Empire low waistline, close skirt, necklace, wrap