Ercole was the youngest of four sons of Lord Honoré I (1522–1581) and Isabella Grimaldi.

His eldest brother Charles II became lord of Monaco on the death of their father in 1581.

Ercole's two elder brothers Francois (1557–1586) and Horace (1558–1559) predeceased Charles, thus leaving Ercole as his eldest brother's heir.

When Charles died without issue in 1589, Ercole succeeded him.

Ercole married Maria Landi on 15 September 1595;[1]: 54  the marriage produced three children; Ercole was murdered in 1604 and his six-year-old son Honoré became Lord of Monaco under the regency of Ercole's brother-in-law, Frederico Landi, 4th Prince of Val di Taro.