The Long Parliament, which was convened in November of 1640, played a revolutionary role in English history. Its greatest overall effect was that it wrested a great deal of political power from the king. Perhaps the most significant accomplishment of the Long Parliament was the passage of the Triennial Act in 1641, which stipulated that Parliament should be convened at least once every three years and could not be dissolved by the King except with the consent of its members. Also, Parliament abolished the two royal courts and declared that bishops of the Church of England could no longer sit in the House of Lords, the legislative body made up of the nobility. All of these actions were taken in a spirit increasingly hostile to the privileges of the monarchy.

Cromwell's appearance and demeanor during this time helped him rise from the backbenches of Parliament to political prominence. Cromwell dressed very simply, in the Puritan manner, and had a commanding voice that was bolstered by his unwavering self-assurance. He was energetic, which enabled him to serve on numerous parliamentary committees without tiring or losing any of his fiery temperament. Although Cromwell started out in the Long Parliament by serving on a number of committees that dealt with church reform, by 1641 he began to take a lead in the reform of other institutions, such as the English treasury, known as the Exchequer. Cromwell also attracted a great deal of attention when he defended poor commoners from the practice known as fen enclosure, the privatization of traditionally communal farmland.

While Cromwell began his parliamentary career on the margins of political power, he became an important ally for the leaders of the House of Commons, most notably John Pym and John Hampden. These men often sent Cromwell on missions to represent the House of Commons in front of the House of Lords, which often favored the King's policies. Pym and Hampden also came to rely on Cromwell's diligence and attention to detail in parliament's business affair, and on the initiative he showed as he moved various committee resolutions through the Commons.

Two key events occurred in November of 1641 that were vital in shaping Cromwell's political career. First, on November one, news of a great rebellion in Ireland reached London. The news sparked fears in many Puritans of a revival of Roman Catholic power in Ireland, and Cromwell himself saw the rebellion as a first step in a frightful Catholic plot to lay waste to England. The rebellion also provided further fodder for the growing animosity between Parliament and King Charles I, primarily over who should have the authority to appoint a commanding officer to quash the Irish rebellion. Cromwell himself was nominated to sit on a new Council for Irish Affairs, and was determined to see Protestant–and Parliamentary–interests prevail.

The second important event was the passage of the Grand Remonstrance on November twenty-two, 1641. The Grand Remonstrance was a bill which spelled out Parliament's grievances against King Charles I, and its passage was a triumph for Pym, Hampden, and their allies, such as Cromwell. The Grand Remonstrance, however, was also the focus of heated and dramatic controversy. On the night of its passage, the bill's supporters and opponents both drew their swords against one another during a session of the House of Commons.

Six weeks later King Charles tried to arrest the leaders of Parliament, but he failed and fled to Scotland to seek support for the royal cause. Over the next few months, it became apparent that war was inevitable, and both Parliament and the King began raising armies for the impending conflict. Cromwell was in the thick of these preparations, pushing motions through the House of Commons for the build-up and financing of an army. He successfully urged Parliament to allow Cambridgeshire and its surrounding areas to raise troops.

In August 1642, King Charles raised the royal standard at Nottingham, marking the beginning of the English Civil War. Under the command of Robert Devereaux, the Second Earl of Essex, Cromwell left London to raise a cavalry troop at Huntingdon. By April of the next year the Huntingdon troop would become an entire regiment, which Cromwell would lead as a colonel. Cromwell imposed strict discipline on his men, and in August 1642 they ambushed a royalist convoy outside of Cambridge. So began Cromwell's rise as one of the key military commanders of the English Civil War.

Popular pages: Oliver Cromwell