He attended Oriel College at Oxford and became a Fellow there. With the accession of Elizabeth I, he refused to take the Oath of Supremacy. Therefore he had to leave England and went into exile on the Continent, joining other Catholics in Leuven in present-day Belgium. Allen returned to England before ordination and began to work with Catholics, recognizing that the religious changes legislated by Parliament were not the will of the people.
So he returned to the Continent and went to another town in Flanders, Mechelen. (When I visited Belgium many years ago, I had no idea of these connections!) There he was ordained in 1565--and never returned to England. Allen founded the college and seminary in Douai for the training of English Catholic priests to return to their native land to serve the Catholic people.
Allen also founded the English College in Rome for the same purpose. He had to move the college at Douai to Rheims--all the while working on an English translation of the Holy Bible. First the New Testament was produced at Rheims in 1582; the Old Testament was delayed until 1609--published two years before King James's Authorized Version.
Those are Allen's uncontrovertible achievements; however, he also dabbled in political and diplomatic efforts. There his record is more controversial as he encouraged the excommunication of Elizabeth I, attempts to remove her from the throne, and the Spanish Armada. It was planned that he would follow the victorious Armada and become the Catholic Archbishop of Canterbury!
When all these plans came to naught, Allen became a Librarian at the Vatican and helped found another English college in Spain. He died in Rome at the Venerable English College.