Lady Mary Fox (née FitzClarence; 19 December 1798 – 13 July 1864) was an illegitimate daughter of King William IV of the United Kingdom by his mistress Dorothea Jordan.

Mary FitzClarence was born in Bushy House as the fourth child and second daughter of the then Prince William, Duke of Clarence and St Andrews, and actor Dorothea Jordan.

Lady Holland worried that she might be "a sickly subject" and wished that the "roturier blood of the mother might have mitigated the royal constitutions".

[3] In January 1857, Sir Frederic Madden, custodian of the manuscripts at the British Museum, learned that Lady Mary wished to sell the roll she had been given by her father in order to raise funds for building a church "or something of that kind".

Later that year, Lady Mary published a utopian feminist Gothic fiction narrative titled An Account of an Expedition to the Interior of New Holland.