It turned out that an antislavery candidate in New York had siphoned off enough votes to give Polk a slim majority. When the results of the 1844 presidential election were tallied, James K. Polk had won with 1,339,368 popular votes and 170 electoral votes to the 1,300,687 popular votes and 105 electoral votes won by Henry Clay. America had elected its first "dark-horse" candidate. Instantly, Columbia–Polk's hometown–became one of the most watched cities in the U.S. Jobseekers and well-wishers arrived in droves. Since Polk had been an unexpected candidate, he owed favors to no one and had made few promises along the campaign trail.

Meanwhile, the issue of Texas' annexation continued to boil. Feelings had grown so heated that in late December a rumor spread that Polk had been killed by an anti-Texan assassin's bullet. In late January, Polk and his wife began their trip to Washington, spending one night at Andrew Jackson's house before "Young Hickory" continued on to assume the presidency via a special steamboat outfitted for the journey. From his first moments in Washington, Polk was busy setting up a new government. He set new high standards for his appointees: They may not stand as candidates for the presidency or campaign for any post while still in Polk's cabinet and must remain in Washington year-round. He appointed James Buchanan as his Secretary of State, William L. Marcy as Secretary of War, and Robert J. Walker as Secretary of the Treasury. He balanced opinions and geographical differences in his selection and even the Whig papers grudgingly acknowledged his fine advisors–although the same papers then predicted that Polk would be too weak to make his own decisions.

After inauguration, Polk found that he quickly had to make decisions regarding Texas and Oregon. America claimed everything south of the 54th parallel (the Alaskan border) in the disputed Oregon territory, although Britain and the U.S. had jointly managed it since a treaty in 1818. Texas, too, proved difficult as both Mexico and England were negotiating with the independent republic. It appeared to be only a matter of time before war began–the only question was with whom the war would be, Britain or Mexico. As Polk's narrow victory showed, the public split evenly on the issues. Now everything was up to Polk, the youngest president in the history of the country.

Polk intended to prove that he did things differently than his predecessors. He started by ending the spoils system, whereby the winning political party appointed new people to fill many of the government's jobs. The President and the First Lady agreed to limit social engagements so that he could focus on his established program: postal reform, reestablishment of the sub-Treasury, tariff reduction, and the annexation of Texas, Oregon and the Mexican province of California. He wanted to adjust the postal rates so that senders paid for postage rather than recipients, and to lower tariffs on necessities while raising tariffs on luxury items.

Events began quickly. The English government criticized Polk for reaffirming the claim to Oregon and Mexico recalled its ambassador. In May 1845, Texas announced that it would hold a convention on July 4 to discuss America's terms for annexation. Texas requested military protection from the U.S. and Polk replied that he had already ordered General Zachary Taylor to the disputed area with three thousand soldiers. Taylor carried with him promises of additional troops, if warranted, and the orders to carry the war into Mexico as soon as war began. However, at the moment, Polk worried more about war with England; he now saw it as all-but inevitable.

Polk ordered U.S. naval vessels into the Gulf of Mexico and in early July he asked Buchanan to reenter talks with England about Oregon. If England actually wanted Texas, then Oregon had to be taken off the table as soon as possible. President John Tyler had offered to compromise on the 49th parallel, but England had rejected it. And Polk was secretly pleased when England again rejected the compromise–he believed he had promised the fifty-four' forty" line, and now must deliver it.

Andrew Jackson, the patriarch of the Democratic Party, died in June 1845. Polk regretted that he could not have lived but a few months longer to see out so many of the plans the old general had laid for the Party.

Mexico realized that any chance it had of invading Texas had evaporated with the arrival of Taylor's troops. They agreed to salvage what the government could through negotiations and communicated the message to the U.S. The news of Mexico's new position arrived shortly after the news that Texans had ratified their annexation. Polk dispatched a negotiating team to Mexico to readjust the U.S.–Mexico border and to purchase New Mexico and California from the Mexican government for up to forty million dollars.