Forbes was born the third and youngest daughter of John Forbes-Sempill, 18th Lord Sempill and 9th Baronet in 1912, christened Elizabeth Forbes-Sempill and known to the family as "Betty". Betty was raised female, but with a childhood dominated by a widespread gender insecurity; after refusing to go away to a girls' school, she was educated at home, and the culmination of this unhappy upbringing was coming out as a debutante in London in the late 1920s. Betty was promptly sent abroad by the family, studying at Munich University, before returning to Scotland and leading a troupe of Scottish country dancers - the "Dancers of Don".
On the death of their father in 1934, both the barony and the baronetcy passed to Betty's elder brother, William, who left the management of the estates in the hands of his sister. Betty took to the new lifestyle with gusto, adopting a broad Doric accent and taking to wearing a masculine kilt.
During the War, Betty studied medicine at the University of Aberdeen, graduating in 1944, and after working at the Aberdeen Royal Infirmary began to work as a general practitioner in Alford, Aberdeenshire in 1945. The area was one of the largest practices in the United Kingdom, and in the winter months Dr. Forbes-Sempill often had to travel through ten-foot snowdrifts in a converted Universal Carrier.
On moving to Alford, Betty began to publicly look and behave like the man he identified as being; in 1952, he formally became male by the simple process of requesting a warrant for birth re-registration from the Sherriff of Aberdeen, registering himself as male and changing his name to Ewan Forbes-Sempill, and publishing a notice to this extent in the Aberdeen Press and Journal of September 12, 1952. His plans had been known in advance to many of his patients, who were universally supportive when contacted by the press; he was equally candid with the press, describing the situation to one reporter as "...a ghastly mistake. I was carelessly registered as a girl in the first place, but of course, that was forty years ago ... the doctors in those days were mistaken, too". Three weeks after, in early October, he announced he was to marry Isabella Mitchell, his housekeeper. He retired from medical work in 1965, to concentrate on farming.
The issue of his gender remained a private one until 1965, when his elder brother Lord Sempill died, with daughters but no sons. The titles were split; the barony passed in the female line to Sempill's eldest daughter Ann, whilst the baronetcy passed to the first male heir. The family had assumed that Ewan would inherit, but this was challenged in court by his cousin John Forbes-Sempill - the only son of Rear-Admiral Arthur Forbes-Sempill, the younger brother of Ewan's father. The Court of Session ruled in favour of Ewan; an appeal was made to the Lord Advocate, who referred the matter to the Home Secretary, James Callaghan, who ruled in December 1968 that Ewan was the rightful holder of the title. On taking up the baronetcy, he changed his name to Ewan Forbes, dropping the Sempill from his surname.
- Obituary in The Daily Telegraph, October 1, 1991. Published in the Daily Telegraph Book of Obituaries, ed. Hugh Massingberd, 1995.
- Peterkin, Tom (30 November 2003), “Sex-change baronet's estate is sold for £1m”, The Daily Telegraph, <http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2003/12/01/nest01.xml>. Retrieved on 26 February 2008