In Huntingdon, Oliver Cromwell took an active interest in local affairs, a practice common among landowning country gentlemen of the time. In 1628, Cromwell was elected to represent Huntingdon in the House of Commons, the legislative body that represented English citizens who were not nobles. He traveled to London that year when Parliament was called. Significantly, during his stay in London Cromwell visited a well-known physician named Dr Theodore Mayerne, apparently because he suffered from a severe case of depression.

Cromwell had to travel to London once again in 1630, this time to argue a case before King Charles I's Privy Council concerning a political dispute over the new town charter of Huntingdon. He was perturbed that he had not been appointed an alderman of the town, and accused the new officials of Huntingdon of pursuing their own selfish interests. The Council reprimanded him for making "disgraceful and unseemly speeches" against the mayor of the town, and Cromwell returned home. Shortly after, in 1631, Cromwell sold all but seventeen acres of his property in Huntingdon and moved his family to a new home five miles away from St. Ives.

At St Ives, Cromwell's life was more that of a yeoman farmer than of a country gentleman. Cromwell's family had suffered a decline in social position when his rich, paternal uncle had lost his fortune at the end of the 1620s. It was not until 1636 that Cromwell would regain his status as a landowning gentleman. That year, an uncle on Cromwell's mother's side passed away and left all his property in Ely, also in East Anglia, to Cromwell. When the Cromwells moved to their new estate in Ely, Cromwell's mother and unmarried aunts joined them.

At some point during the late 1620s or early 1630s, Cromwell experienced a momentous spiritual conversion. Previously, despite his Puritan upbringing, Cromwell had not been terribly fervent in his Christianity. A letter he wrote to a cousin in 1638, however, shows how profoundly his new religious convictions had overtaken him: "My soul is with the congregation of the firstborn, my body rests in hope," Cromwell wrote. "The Lord accepts me in His Son, and gives me to walk in the light, as he is in the light. Blessed be His name for shining on so dark a heart as mine!" Like a number of other devout Puritans at the time, Cromwell came to see himself as one of God's Elect, the select few who, according to Calvinist doctrine, were predestined for eternal salvation in heaven. This conviction fired Cromwell with a great sense of destiny and personal righteousness.

Cromwell's newly charged Puritanism greatly affected his political views. In early 1640, Cromwell was elected once again to sit in Parliament, this time as a representative for Cambridge. This year marked the beginning of the Long Parliament, and from the beginning Oliver revealed himself to be one of the strongest defenders of Parliament in opposition to the actions of King Charles I. For Cromwell, this opposition was centered on the defense of the Protestant Reformation in England, and he sat on a number of parliamentary committees that dealt with cases of church reform. He argued his views with considerable fire and zealotry, at one point even proposing a bill "for the abolition of superstition and idolatry and for the better advancement of true worship and service of God."

In his zeal for Protestant reform in the early days of the Long Parliament, Cromwell was unable to gain much respect or favor from his colleagues. The leaders of Parliament looked upon him as naive, excitable, and extremely impulsive. At this time he was very much a backbench Minister of Parliament, or MP. His position among his colleagues was low, and his loud railings against King Charles I and his allegedly pro-Catholic court went unheeded as embarrassing ravings.

Cromwell was not, however, the only MP to nurse such strong convictions. Other Puritan MPs shared Cromwell's devotion and conviction that they were members of the Elect. Although few other MPs were as vocal as Cromwell, many shared his concern that Charles I and his government threatened the goals of the Protestant Reformation. This militant faction of Parliament saw Henry VIII's and Elizabeth I's earlier reformations of the English Church as half-hearted measures at best. These earlier monarchs had created a Church of England separate from the Catholic Church, but now the Puritan MPs wished to purge the Church of England of all Catholic elements, such as landed bishoprics, the wearing of expensive vestments by the clergy, and many other rituals that they thought made the Church of England too similar to the Catholic Church.

King Charles I presented a sizable obstacle to the goals of Parliament's Puritan faction. In 1633, for example, Charles appointed William Laud, whose views were far closer to Catholic doctrine than they were to Calvinism, as Archbishop of Canterbury, the most powerful position in the Church of England. Furthermore, Charles's queen, Henrietta Maria, was a Roman Catholic. To the Parliamentarians' dismay, Henrietta Maria and many foreign ambassadors–including papal envoys welcomed by Charles into his court–were permitted to hold Catholic masses.

In addition to these actions, which the Puritan MPs characterized as a "Popish Plot", Charles also distressed Cromwell and his colleagues by refusing to summon Parliament regularly. Instead, Charles made repeated attempts to increase taxes without the customary sanction of Parliament. The Long Parliament, which convened in November 1640, wished to curb Charles's power in these matters, and the resultant controversies were the first steps toward the English Civil War.