Marcello Mastroianni was the original choice for the crown Prince while Alec Guinness was being pursued to play his father.
Originally planned as a starring vehicle for the husband-and-wife team of Mel Ferrer and Audrey Hepburn, who had performed Mayerling for TV in 1957.
Mel Ferrer had planned to produce, and director Terence Young had worked with his wife Audrey Hepburn in Seule dans la nuit (1967) the previous year. Ferrer was still listed as producer up to 1967 but by the time production began, the Hepburn/Ferrer marriage was no more, and Ferrer was no longer involved.
Omar Sharif would later make a cameo appearance in the mini-series, Prince Rodolphe : L'Héritier de Sissi (2006), which also deals with the same events as Mayerling.