Lady Mary is depicted as St Agnes in a rocky cave, holding a palm branch in her left hand and resting her right hand on a lamb. St Agnes was the patroness of those about to be married and the portrait was probably painted on the eve of her marriage on 3 August 1637 to the Duke of Richmond. However, she perhaps looks a little older than fifteen.

The head and hands are very sensitively painted and the dress is fashionable, but includes fanciful details, such as the jewels, sleeves and scalloped edges to the sleeve and skirt. The cold tones, noticeable in the sky are characteristic of Van Dyck’s later work.

Lady Mary was the only daughter of the murdered Duke of Buckingham and was much beloved of the royal family. Her first marriage to Lord Herbert (d.1636) took place in the Royal Closet at Whitehall in 1635 and her marriage to James Stuart, Duke of Richmond and Lennox (d.1655) in the Archbishop’s Chapel at Lambeth, where she was given away by the King. She was Lady of the Bedchamber to Catherine of Braganza. Her third marriage was to Col. Thomas Howard.

The painting appears in Pyne's illustrated 'Royal Residences' of 1819, hanging over the mantle in the Queen's Ball-Room at Windsor Castle (RCIN 922101).