He was born in the principality of Monaco, the only son of Princess Charlotte Louise Juliette of Monaco (1898 - 1977) -- the illegitimate and later legitimated Algeria-born daughter of Louis II of Monaco and Marie Juliette Louvet, reportedly either a cabaret singer or a laundress -- and her husband, Prince Pierre-Marie-Xavier-Antoine-Melchior of Monaco (1895 - 1964), né Prince Pierre de Polignac. He had one sister, Princess Antoinette.
Rainier was first sent to study at St. Leonards-on-Sea in England, then at Stowe, a prestigious private English school. From there, he went to Institute Le Rosey, Rolle/Gstaad, Switzerland before going on to university in France.
His grandfather, Prince Louis II, had been a General in the French army and during World War II. Rainier served as an artillery officer in the army. Second Lieutenant Rainier fought so courageously during the German counter-offensive in Alsace that he won the "Croix de Guerre," the "Bronze Star" and was given the rank of "Chevalier in the Legion of Honor."
He is the world's second longest reigning monarch.
In the years since his wife's death, he was reportedly involved with a distant cousin, Princess Ira von Fürstenberg, a former movie actress turned jewelry designer who is also a Fiat heiress and the former sister-in-law of fashion designer Diane von Furstenberg. Like him, she is a great-grandchild of Lady Mary Douglas-Hamilton, the Scottish-Bavarian wife of Prince Albert I of Monaco, though by Lady Mary's second marriage.