In 1839, Lincoln met Mary Todd in Springfield, Illinois, and the following year they became engaged. She was the daughter of Robert Smith Todd, a wealthy lawyer and businessman in Lexington, Kentucky. A wedding set for January 1, 1841, was canceled at Lincoln's request, but they reconciled and married on November 4, 1842, in the Springfield mansion of Mary's sister. While anxiously preparing for the nuptials, he was asked where he was going and replied, "To hell, I suppose." In 1844, the couple bought a house in Springfield near his law office. Mary kept house with the help of a hired servant and a relative.

Lincoln was an affectionate husband and father of four sons, though his work regularly kept him away from home. The oldest, Robert Todd Lincoln, was born in 1843 and was the only child to live to maturity. Edward Baker Lincoln (Eddie), born in 1846, died February 1, 1850, probably of tuberculosis. Lincoln's third son, "Willie" Lincoln was born on December 21, 1850, and died of a fever at the White House on February 20, 1862. The youngest, Thomas "Tad" Lincoln, was born on April 4, 1853, and survived his father but died of heart failure at age 18 on July 16, 1871.Lincoln "was remarkably fond of children" and the Lincolns were not considered to be strict with their own. In fact, Lincoln's law partner William H. Herndon would grow irritated when Lincoln brought his children to the law office. Their father, it seemed, was often too absorbed in his work to notice his children's behavior. Herndon recounted, "I have felt many and many a time that I wanted to wring their little necks, and yet out of respect for Lincoln I kept my mouth shut. Lincoln did not note what his children were doing or had done." The deaths of their sons, Eddie and Willie, had profound effects on both parents. Lincoln suffered from "melancholy", a condition now thought to be clinical depression.