James became king of Scotland as James VI in 1567 after Mary, Queen of Scots, was compelled to abdicate. Regents governed during his childhood, and rival nobles continued to contest influence over him after he began exercising authority. He gradually strengthened the monarchy by managing aristocratic factions and negotiating a difficult relationship with the Presbyterian church. His writings, including The True Law of Free Monarchies and Basilikon Doron, defended a strong royal office while also describing a ruler's duties. Daemonologie reflected his involvement in the witchcraft controversies of his Scottish reign.

In 1603 he inherited the English and Irish crowns after Elizabeth I's death, becoming James I in England. The succession united the kingdoms under one sovereign, but did not merge their parliaments, legal systems or churches. James favoured a closer union and the designation Great Britain, yet encountered resistance to full integration. He made peace with Spain in 1604, ending the long Elizabethan war. At the Hampton Court Conference he authorized a new English Bible translation, published in 1611. His court supported theatre and architecture, including Inigo Jones.

Religious and financial tensions limited his ambitions. The Gunpowder Plot of 1605 led to renewed restrictions on Catholics, while Puritans pressed for further Protestant reform. In Scotland his attempts to promote bishops and prescribed forms of worship provoked opposition. The crown's debts, court expenditure and disputed methods of raising money repeatedly brought conflict with Parliament. The proposed Great Contract of 1610 failed, and the short Parliament of 1614 passed no legislation. Royal favour to figures such as Robert Carr and George Villiers, later Duke of Buckingham, made patronage and access to power especially contentious.

Overseas, his reign saw the establishment of Jamestown in Virginia and the Plantation of Ulster, enterprises with lasting consequences for Indigenous peoples and Irish landholders. During the Thirty Years' War he sought to protect his daughter's family and restore his son-in-law's position without committing immediately to a major European war. Negotiations for a Spanish marriage for Prince Charles collapsed, weakening that policy. By James's death in March 1625, England was moving toward war with Spain. He left a secure dynastic succession to Charles I but unresolved disputes over revenue, religion and the scope of parliamentary debate.