Mabel Purefoy FitzGerald's siblings were Admiral (Royal Navy) Richard Purefoy FitzGerald, later Purefoy (1862-1941, married Mary Lillias Sandys Lumsdaine), Geraldine Mary FitzGerald (1863-1900), Mary Frances Purefoy FitzGerald (1864-1951), Laura Purefoy FitzGerald (1865-1942), Rev. Henry Purefoy FitzGerald (1867-1948, married Lilian Mary Langton), and Catharine 'Katie' Purefoy FitzGerald (1870-1953).

Mabel Purefoy FitzGerald's father was Lieutenant (Royal Navy) Richard Purefoy FitzGerald (1837-1895), her mother was Henrietta Mary, nee Chester (d. 1895); paternal grandparents Thomas FitzGerald (1788-1860) and Sarah Anna Elizabeth ('Eliza'), nee Purefoy Jervoise (1809-1899); maternal grandparents Rev. Anthony Chester (1800-1856) and Henrietta, née Brown (d. 1854).

Other notable family members include: Sarah Anna Elizabeth FitzGerald's' father Major (1st Royal Dragoons) Richard Purefoy Jervoise (1778-1811), her uncle George Purefoy Jervoise, M.P. for Salisbury and for Hampshire (1770-1847) from whom she inherited the Purefoy family estate in Shalstone; her children Captain (1st Royal Dragoons) Thomas Keane FitzGerald (1833-1887), Robert Allan FitzGerald (barrister-at-law, cricketer and Secretary of the MCC, 1834-1880), Captain (16th Lancers) Maurice George FitzGerald (1837-1901), Geraldine FitzGerald (1840-1850), Henry Jervoise FitzGerald (1842-1852), Mary FitzGerald (1845-1860), and Alfred William FitzGerald (1849-1871).

Keane FitzGerald (Barrister of the Inner Temple, 1748-1831) was a relative [uncle?] of Thomas FitzGerald.