Leonard Calvert (1606 - 1647) was the younger son of George Calvert, 1st Baron Baltimore. When his older brother Cæcilius Calvert, 2nd Baron Baltimore received a charter for the colony of Maryland in 1632, Leonard was appointed the colony's first governor. He landed there in the spring of 1634, arriving with 17 gentlemen and their wives and about two hundred others aboard the ships Ark and Dove and establishing the town of St. Mary's. As per his brother's instructions, he at first attempted to govern the country under feudalistic precepts but in February 1635 he had to summon a colonial assembly. In 1638 the assembly forced him to govern according to the laws of England, and subsequently the right to initiate legislation passed to the assembly.

In 1638 Calvert seized a trading post in Kent Island established by the Virginian William Claiborne. In 1644 Claiborne led an uprising of Maryland Protestants. Calvert was forced to flee to Virginia, but he returned at the head of an armed force in 1646 and reasserted proprietorial rule.