Although Polk was a religious man, his faith seldom equaled the stern beliefs of Sarah's outspoken devotion. Raised a Presbyterian, Polk had never been baptized due to an early family argument with the local Presbyterian minister in rural North Carolina. At age thirty-eight, Polk had a religious conversion to Methodism at a camp meeting, and thereafter he thought of himself as a Methodist. Out of respect for his mother and wife, however, he continued to attend Presbyterian services. But whenever his wife was out of town, or too ill to attend church, Polk worshiped at the local Methodist chapel. On his deathbed, he summoned the man who had converted him years before, the Reverend John B. McFerrin, who at last baptized Polk as a Methodist.

Never having children, Polk had no family life as President other than what Sarah would arrange for him. His family was politics, and he pursued it relentlessly. His biographer writes that aside from politics, Polk "had no aspirations, intellectual interests, recreation, or even friendships." He once described himself as the hardest-working man in Washington.