Frederica Mildmay, Countess of Mértola

Frederica, Countess FitzWalter (circle of Joseph Highmore, 1727)

Frederica Susanna Mildmay, (née Schomberg), Countess FitzWalter, 3rd Countess of Mértola (Berlin, 1687 – 7 August 1751) was a British peeress.

Frederica was the eldest surviving daughter of Meinhardt Schomberg, 3rd Duke of Schomberg and his second wife, Karoline, a daughter of Charles I Louis, Elector Palatine. On 26 May 1715, she married the 3rd Earl of Holderness and they had two surviving children: Hon. Robert (1718–1778) and Lady Caroline (d. 1778, married the 4th Marquess of Lothian).

On the death of the countess' father in 1719, she inherited his Portuguese countship of Mértola as his eldest surviving child. Her husband died in 1721 and on 18 June 1724, she married Hon. Benjamin Mildmay, who was later created Earl FitzWalter in 1730.

Via cognatic primogeniture, she was the most senior descendant of James I of England to be Protestant, but she was passed over for Sophia of Hanover. Her cognatic heir is Anthea Theresa Lycett (born Marcia Anne Miller), Countess of Mértola, eldest daughter of the late Diana, Baroness Conyers and Fauconberg and Countess of Mértola.