He was the son of Sir Henry St John-Mildmay, 3rd Baronet of Dogmersfield Park and Jane Mildmay, the daughter and co-heiress of Carew Mildmay of Shawford House, Hampshire.

[1] He was educated at Winchester School from 1798 to 1802, and Christ Church, Oxford, graduating in 1805.

Before her death following childbirth, they were the parents of one son:[1] In 1815, five years after Charlotte's untimely death from giving birth to their son, he eloped with her sister Harriet, Countess of Rosebery (née Bouverie), the wife of Archibald Primrose, 4th Earl of Rosebery in Stuttgart, Württemberg.

Before Harriet eventually left him, they had three sons:[1] Plagued by financial problems, he shot himself on 17 January 1848.

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