By his industry and judgement Calvert won the confidence of the King, was knighted in 1617, was chosen one of James's two secretaries of state. Meanwhile he served several terms in Parliament. His knowledge and integrity brought him the respect of all about the court. For these qualities as well as for his able and faithful services, King James and, later, Charles I rewarded him with substantial grants. James gave him an annual pension of £1000 and extensive lands in Country Longford, Ireland. In 1625, however, Calvert's career reached a turning point when he had announced that he had become a member of the Roman Catholic Church, an event that in England of the seventeenth century was likely to put an end to political preferment. In February, Calvert resigned his office, whereupon James created him Baron Baltimore in the Irish peerage.

Calvert's zeal for American colonization was foreshadowed early in his career when he became a member of both the Virginia and the New England companies. In 1621 he purchased lands in Newfoundland, where he soon planted a settlement, later erected by royal patent into the province of Avalon. He twice visited the colony, but in 1629 the severe climate drove him to ask for lands farther south. After a journey to Virginia he returned to England and petitioned Charles I for a grant adjacent to that colony. With the sanction of the King, Calvert drew up a charter conveying to himself broad holdings on both sides of the Chesapeake Bay, as far as the fortieth degree of latitude. Before the instrument passed the seals Calvert died, but it was a document which, when approved on June 20, 1632, became the means of translating into reality the first Lord Baltimore's dream of American colonization. In the charter the name of Maryland ("Terra Mariae") appeared for the first time.

The portrait is by Daniel Mytens the elder, court painter to both James I and Charles I.