The poem is set in the late 18th century, during a period of unrest in England. While the poem does not explicitly state the reign of a specific King George, it is likely to be during the reign of King George III, as he was the king during this period (1760-1820).
However, the poem doesn't rely on a specific historical King George to enhance its story. It is more focused on the romance and tragedy of the main characters, the highwayman and Bess, and the poetic imagery of their love and sacrifice.