On the female line, he had some blood of Plantagenet Kings of England (Plantagenet - Mortimer - Wroth - Mynne - Calvert). Leonard's wife, Anne Brent, also descended from Edward III (Plantagenet - Beaufort - Neville - Willoughby - Greville - Reed - Brent). Anne Brent's linage is documented in Volume 4 of "Living Descendants of Blood Royal" published by World Nobility and Peerage. Anne was an Eleventh generation descendant from King Edward Third of England. Her G/G/grandmother was Margaret Neville, and so on back to Edward.
On June 20, 1632, Cecil, the newly titled, Lord Baltimore executed the charter for the colony of Maryland that his father had negotiated. The charter consisted of 23 sections, but the most important conferred on Lord Baltimore and his heirs besides the right of absolute ownership in the soil, certain powers: ecclesiastical as well as civil resembling those possessed by the highest nobility handed down from the Middle Ages. Leonard was appointed the colony's first Governor.
Two vessels, the Ark and Dove, carrying over 300 settlers sailed from the harbour of Cowes, 22 November, 1633, arriving at Point Comfort at the mouths of the James, Nansemond and Elizabeth Rivers, in Virginia, 24 February, 1634. On 27 March they landed at what is now St. Mary's, then the site of an Native American village, and they began the work of establishing a settlement.
As per his brother's instructions, Leonard at first attempted to govern the country in an absolutist way but in January 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. In 1643 Governor Calvert went to England to discuss policies with the proprietor, leaving the affairs of the colony in charge of acting Governor Brent. Leonard Calvert married Ann Brent, daughter of Richard Brent. Later in 1643, Ann gave birth to a son, William Calvert. Leonard Calvert returned to Maryland in 1644 with his wife and child, but was soon forced to flee to Virginia. He returned at the head of an armed force in 1646 and reasserted proprietorial rule.
Leonard Calvert died of an illness in the summer of 1647. Before he died, he wrote a will naming Margaret Brent the executor of his estate.
In 1890 the state of Maryland erected a monument to him and his wife at St. Mary's.