Mary Buchanan was a friend of Richard Temple and Anna Eliza Brydges, who called her 'Bucky.' 

Mary's ancestry and biography is not known. She appears to be parentless by 1793, when she was living with Sir Richard Gamon and his first wife (BRO D-FR/46/7/5; 20 September 1793). At the death of Gamon's wife in 1794, Nugent Buckingham offered to take Mary in at Stowe:

Poor Gamon! don’t you feel for him? Yet I think his grief to violent considering that he must have expected the event so long, if any thing I think Bucky bears it worse than Gamon; indeed the loss is a very serious one for her. When Mrs Gamon died Father initially offered to take Bucky into his house till some plan might be fixed upon for her; Gamon however said that she had been entrusted to his care by her Father & therefore could not think of parting with her. She is now therefore to live for the future at Minchenden where the Duchess of Chandos is, & the only society she will have, will be the two Miss Williamson’s who attend the Duchess [...] & the old Housekeeper. I must confess I think Gamon perfectly in the right in his Ideas, yet it must be a melancholy prospect for a Girl like Bucky who well all know is fond of society & of seeing the World. (Richard Temple to William Henry Fremantle; BRO D-FR/46/8/19, 21 August 1794.)

In 1795, Mary is in Dublin (HEH STG Correspondence Box 68 (41)). There is no further mention of her in the Huntington or Fremantle collections.