James Graham inherited the earldom of Montrose from his father in 1626. He was educated at St. Andrews University where he became inspired by classical tales of military glory in writers such as Caesar, Xenophon and Lucan. In November 1629, he married Magdalene Carnegie, daughter of Lord Carnegie of Kinnaird. After the birth of his first two sons, Montrose went to France and Italy to complete his education, which included a period at the French military academy at Angers.
He returned to Scotland in 1637 and became active in the revolt against the imposition of Archbishop Laud's prayer book on the Scottish Kirk. Montrose signed the National Covenant in 1638, and sat in the Glasgow Assembly, which abolished episcopacy and established presbyterian church government in Scotland. The King's representative, the Marquis of Hamilton, noted Montrose's assertiveness and enthusiasm, but regarded it as vanity.
Montrose gained his first military experience leading Covenanter troops in the First Bishops' War. He drove the Royalist Marquis of Huntly out of Aberdeen in March 1639 and campaigned against Huntly's clan, the Gordons. But in June, Huntly's son, Viscount Aboyne, sailed into Aberdeen harbour in one of the King's warships and trained his guns on the town. Surrounded by hostile clansmen, Montrose withdrew to gather stronger forces. He returned three weeks later with artillery and bombarded Aberdeen from the Brig o' Dee until Aboyne and the Gordons fled the city.