He was born at Stemnitsa, Arcadia, but he grew up at Zakynthos.

He was a son of Theodoros Kolokotronis and his mother was Aikaterini Karousou (Greek: Αικατερίνη Καρούσου).

He acquired the nickname "Gennaios" (meaning "brave") during the Greek War of Independence in which he fought valiantly despite his youth.

Kolokotronis served as the aide-de-camp of King Otto with the rank of Major General, and was appointed by Otto as his last Prime Minister at 1862.

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