A historian, a scholar and a key figure in the religious debates of the 16th century, Ussher was born in Dublin (either on Nicholas St or 57 High St across from the old Christchurch Synod Hall) on the 4th January 1581 and baptised in the Church of Ireland St Nicholas Within. He was born into a prosperous merchant family, but one that was torn by the religious divisions of the time. Most of the Usshers conformed to the Church of Ireland, notably Henry Ussher, Church of Ireland archbishop of Armagh from 1595 to 1613, who played an important role in the creation of Trinity College, Dublin in 1592. His mother remained Catholic; her brother was the famous historian Richard Stanyhurst (1547-1618), in later life a Jesuit priest .
James Ussher was one of the first to attend Trinity College Dublin, gaining his BA c. 1597 and MA in 1601. He was ordained by his uncle Henry Ussher in December 1601 and in 1602 took his first trip of many to England, in search of books for Trinity College Library. During his trips to London he made many antiquarian connections with figures such as Henry Savile and John Seldon. He graduated in Divinity in 1607, immediately becoming Professor of Theological Controversies. In 1613 he married Phoebe Challoner, daughter of the vice-provost of Trinity Luke Challoner, and published his first work, Gravissimae Quaestionis, de Christianarum ecclesiarum, in occidentis praesertim partibus, ab apostolicis temporibus, ad nostram usque aetatem, continua successione & statu, historica explicatio (London, 1613). This work gave an account of medieval heretical groups drawing on original sources; however it also had the polemical purpose of tracing the rise of the Anti-Christ in Roman Catholic church, and the identification of groups such as the Cathars as proto-protestants.
Ussher graduated DD in 1614 and became vice-chancellor of Trinity in 1615. He is also credited with playing a large part in the creation of the Church of Ireland’s first full Confession of Faith. These articles were an advance on the 39 Articles on which they were based, they allowed more accommodation of puritan ideas than the equivalent in the Church of England (for example, allowing more leeway on the question of bishops) and did not require subscription. Ussher became Bishop of Meath and Clonmacnoise in 1621 and finally was appointed Archbishop of Armagh by James I of England in 1625. It should be noted that of the twenty-five bishops James I placed in the Church of Ireland in his lifetime, Ussher was the only one born in Ireland and educated at Trinity