In 1836, he began to study law under attorney Bradford Russell, whose office was above the store where he clerked. Boutwell did not take the bar exam or enter into active practice until many years later. In 1838, the shop owner offered Boutwell a partnership in the shop. While Boutwell ran the store, he began a personal regimen of reading and writing in an effort to make up for having chosen not to attend college. Boutwell made his public career debut in 1839, when he served as a pension agent for widows of the American Revolutionary War, which had ended in 1783. He traveled to Washington D.C. and was impressed after seeing Daniel Webster. After talking with a black slave woman whose youngest child had been sold to Louisiana, Boutwell became dedicated to the anti-slavery cause. Boutwell married Sarah Adelia Thayer on July 8, 1841. Sarah was the daughter of Nathan Hayler from Hollis, New Hampshire. Their marriage produced two children: Georgianna (May 18, 1843) and Francis (February 26, 1847). Entering politics as a Democrat and supporter of Martin Van Buren, Boutwell was appointed head of the Groton post office by his business partner, who had been appointed postmaster. Boutwell's first entry into elective politics was a successful run for the Groton School Committee as a Temperance Party candidate; he would sit on that committee for many years. The success prompted him to run for the state legislature on the same party's ticket; because the party was a small third party, he lost badly. In 1840, he won the Democratic Party nomination, despite temperance opinions that were "offensive to many", but lost in a Whig landslide. He finally won on the third try, defeating incumbent John Boynton in 1841. He won reelection twice before being defeated in 1844. Although he also lost in 1845, he was returned to the state legislature in the 1846 election, serving from 1847 to 1850. His elective successes, sometimes in the face of major Whig victories statewide, highlighted Boutwell's potential, and brought him into the Democratic Party's leadership circles. He sat on the judiciary and finance committees, where he gained a reputation for thorough research into legislation, and advocated positions favoring free trade, restraint of the money supply, and increased taxes for spending on education and other reforms. He supported the Mexican–American War, which (unlike others) he did not view as a major slavery-related issue.