Polk had arrived on the Tennessee scene just as the nation plunged into economic crisis with the Panic of 1819. His law mentor, Felix Grundy–an up and coming politician–helped Polk secure the post of clerk of the House of Representatives, a post that Polk would hold for four years. In 1821, Polk arrived in Murfreesboro for the next session of the legislature (Tennessee at that time had no permanent state capital; it rotated between Knoxville, Kingsport, Nashville and Murfreesboro). The budding national prominence of Andrew Jackson had invigorated the whole state, for it appeared that Jackson was heading for Washington and the presidency. Grundy stood as one of Jackson's closest advisors and friends. As a former Chief Justice of the Kentucky Supreme Court and a Tennessee congressional representative, Grundy's words had much impact, and his friendship opened many doors for Polk in his early political days.

Early in the legislative term, the government celebrated with a gala event for the state's governor where he met Sarah Childress, the daughter of a wealthy and prominent family, whom Polk had briefly known while they were at the academy several years prior. Within a few months, his calls at the Childress house had become daily occurrences. Sarah encouraged Polk to run for the legislature himself, and he declared his candidacy soon thereafter.

Polk began his political campaign with a horseback tour of Maury County, where he listened to complaints and answered questions as best he could. With little in the way of entertainment in backwoods Tennessee, the arrival of a young politician often became the center of much attention: Merchants closed shops, farmers came in from the fields and families brought picnics to listen to the speeches. In November, Polk, who was now known as the "Napoleon of the Stump," won handily and he next entered the House of Representatives not as a clerk but as a full representative.

First on the new legislature's agenda was its response to the internal improvement plan set forth by President James Madison. Madison's plan to spend Federal money to build roads and canals throughout the states met strong resistance in some areas as opponents feared that the federal roads would diminish states' rights. As Polk asked, "Shall we jeopardize our sovereignty for the sake of a dirt road and a canal or two?" Although he admitted that Tennessee–like many other states–was too poor to build the roads himself, he did not believe the trade-off was worth it. Eventually, Tennessee sent its senators off to Washington with the caveat to proceed carefully and to ensure that state independence remained protected under any plan. Polk loved every minute of the debate, and he remarked to Sarah, now his fiancée, that his only regret about entering politics was that he "didn't take the plunge sooner." On January 1, 1824, he and Sarah married. Although they would have a loving and strong marriage for many years, they never had children.

The next year, Polk hit the campaign trail again–this time for the U.S. House of Representatives. The presidential election in 1824 had left bitter feelings in Tennessee, as John Quincy Adams had secured the presidency after the election had been thrown into the House of Representatives. Andrew Jackson had won in the Electoral College, but had not gained a majority. Adams had arraigned it so that the Speaker of the House, Henry Clay, who also had been a candidate, would drop out and throw his support behind Adams in exchange for being appointed Secretary of State. When the votes were tallied Adams won, and in the coming weeks, the Adams-Clay deal became clear and Jackson and his supporters cried foul over the "corrupt bargain." In their eyes, this wrong had to be avenged. In October, leaving Sarah behind, Polk began his long trip to Washington.

Washington at that time remained little more than a shell of the great city that it had been designed to be. Besides a few large government buildings, most of the city remained swamp and good accommodations were hard to come by. Polk arrived in Washington just barely thirty years old and he spent his first two months in Congress listening and watching. On January 23, 1826, Polk presented a bill to authorize the Federal government to relinquish certain lands in Tennessee. Seven weeks later, on March 13, Polk made his "maiden speech" to the House in support of a bill that would disassemble the Electoral College in order to prevent a rerun of the 1824 election–the power of the election must be in the hands of the people, Polk proclaimed. A month later, Polk again visited the floor to oppose the Panama Mission, in which the U.S. had been invited to send delegates to a Panama convention with other western nations to discuss better relationships. Polk feared that the mission would lead to "entangling alliances." After weeks of debate, Congress approved the mission–only to have it fizzle before it really began: one delegate died en route and the other delegate arrived only after the conference had wrapped up. The potential problems had been averted.