Georgina Thomas was born on 24 May 1837, a day of general rejoicing throughout Great Britain. Church bells rang; schools and shops were closed; and there were firework displays, tea parties and public dinners with speeches and toasts. It was ‘a day that the old would talk about for a long time, and the young would never forget’. This had, however, nothing whatsoever to do with Georgina, who happened to share her birthday with the young heir to the throne, Princess Victoria. In May 1837 Victoria turned eighteen and, under the provisions of the Regency Bill of 1831, attained her ‘royal majority’. This meant that she was able to rule alone, in her own right, when her uncle William IV died, less than a month after her birthday. The knowledge that she had been born on such an important day gave Georgina a ‘vague idea of superiority and of relationship to the Royal Family’ from an early age. This feeling was reinforced by the erroneous belief that the Thomases were descended from Edward III and would be entitled to claim the throne of England ‘if anything should happen to the reigning family’.

Georgina was the second daughter of Morgan Thomas of Gate House in the parish of Mayfield in Sussex and his wife, Louisa Frances Dalrymple. An elder sister, Cordelia, died of whooping cough when she was just seventeen months old, a few weeks after Georgina's birth. Morgan and Louisa had convinced themselves that their second child would be the longed-for son who was to continue the family line, and they made no attempt to hide their disappointment when the new baby turned out to be a girl. Georgina always felt that her parents had been dissatisfied with her since the day of her birth, somehow blaming her for her sister's early death. As Louisa wept beside Cordelia's empty cot, Georgina (with, as she later wrote, her habitual lack of tact) ‘kicked and screamed with life and joy’.

The Thomas family was of Welsh origin. There is no evidence for their supposed royal descent, but they could trace their ancestry back in the male line to one Traherne ap Thomas, who was living at Lletty Mawr in Llannon, Carmarthenshire, in 1597.