William I, Elector of Hesse (German: Wilhelm I., Kurfürst von Hessen; 3 June 1743 – 27 February 1821) was the eldest surviving son of Frederick II, Landgrave of Hesse-Kassel (or Hesse-Cassel) and Princess Mary of Great Britain, the daughter of George II.

Prince William was born on 3 June 1743 in Kassel, capital of the Landgraviate of Hesse-Kassel in the Holy Roman Empire.

His father's marriage with the British princess was not a happy one, and Frederick abandoned the family in 1747 and converted to Catholicism in 1749.

During the lifetime of his father, William had already received the Principality of Hanau, south of the Hessian territories near Frankfurt, as successor of its newly extinct princes.

The early fortunes of the Rothschild family were made through a conjunction of financial intelligence and the wealth of Prince William.

This money then saw its way through to Nathan Mayer, (N.M.) in London, where it helped fund the British movements through Portugal and Spain.

The interest made from this venture was reaped by the budding banker barons, who used it to swiftly develop their fortune and prestige in Europe and Britain.

However, the European powers refused to recognize this title at the Congress of Aix-la-Chapelle (1818) and instead granted him the grand ducal style of "Royal Highness.

"[3] Deeming the title of Prince-Elector to be superior in dignity to that of Grand Duke, William chose to remain an Elector, even though there was no longer a Holy Roman Emperor to elect.