Francis Stephens, the Duke of Lorraine, received the first two Masonic degrees in 1731 in a special lodge convened at The Hague, Holland, lead by John Theophilus Desaguliers.
In doing so, he is thought to be the first known royal Freemason. Later he received the third degree in England. In 1745, he ascended to the throne, becoming Francis I, Holy Roman Emperor, in part due to his marriage to Maria Theresa of Austria. (Source: Porchway Lodge No. 7027, UGLE)