Lyman Trumbull (October 12, 1813 – June 25, 1896) was an American lawyer, judge, and politician who represented the state of Illinois in the United States Senate from 1855 to 1873. Trumbull was a leading abolitionist attorney and key political ally to Abraham Lincoln and authored several landmark pieces of reform as chair of the Judiciary Committee during the American Civil War and Reconstruction era, including the Confiscation Acts, which created the legal basis for the Emancipation Proclamation; the Thirteenth Amendment to the United States Constitution, which abolished chattel slavery; and the Civil Rights Act of 1866, which led to the Fourteenth Amendment to the United States Constitution.

Born in Colchester, Connecticut to a prominent political family, Trumbull studied law in Greenville, Georgia, before moving to Illinois to establish a practice and enter politics. He served as the Illinois Secretary of State from 1841 to 1843 and as a justice of the Illinois Supreme Court from 1848 to 1853. As an attorney, Trumbull successfully argued the case Jarrot v. Jarrot, which de facto banned slavery in the state.

In 1855, Trumbull was elected to the Senate as the choice of the anti-slavery faction of the Illinois legislature, defeating Abraham Lincoln. Lincoln endorsed Trumbull for the election; the two soon became leading members of the new Republican Party. After the American Civil War, Trumbull was a leading moderate Republican, favoring both civil rights for freed slaves and reconciliation with the South.

In the 1868 impeachment trial of President Andrew Johnson, Trumbull voted to acquit Johnson despite heavy pressure from other Republican senators. He broke with the Republicans in 1870 and was a candidate for the presidency at the 1872 Liberal Republican convention. After returning to the Democratic Party, Trumbull left the Senate in 1873 to establish a legal practice in Chicago. Before his death in 1896, he became a member of the Populist Party and represented Eugene V. Debs before the Supreme Court of the United States.

Lyman Trumbull was born in Colchester, Connecticut on October 12, 1813, to Connecticut's leading political family, which included three Governors and had arrived in the Massachusetts Bay Colony from Newcastle upon Tyne in 1639. His father, Benjamin Trumbull Jr., was an attorney, farmer, state representative, and the son of the historian Benjamin Trumbull. His mother, Elizabeth Mather, was a member of the Mather family of prominent New England Congregationalist clergymen including Increase Mather and Cotton Mather. Lyman was the seventh of eleven children, eight of whom survived into adulthood.

Trumbull attended Bacon Academy in Colchester, where he studied a traditional course in math, Latin, and Greek. When he turned eighteen, he teaching in Portland, Connecticut; New Jersey and Colchester. In 1833, he traveled to Pike County, Georgia, in hopes of becoming a schoolteacher there. Finding no position available, he proceeded to Greenville, where he was hired as principal of the Greenville Academy. Trumbull remained at Greenville for three years, where he read law in the offices of Hiram B. Warner. In 1837, he moved to Belleville, Illinois, where he began a legal practice in the office of John Reynolds, the former Governor of Illinois.

While living in Belleville in November 1837, Trumbull became aware of the murder of Elijah P. Lovejoy, an abolitionist minister and newspaper publisher, in nearby Alton. In a letter to his father, the young Trumbull predicted, "[Lovejoy's] death and the manner in which he was slain will make thousands of abolitionists, and far more than his writings would have made had he published his paper an hundred years. … As much as I am opposed to the immediate emancipation of the slaves and to the doctrine of Abolitionism, yet I am more opposed to mob violence and outrage, and had I been in Alton, I would have cheerfully marched to the rescue of Mr. Lovejoy and his property."

In 1840, Trumbull was elected from St. Clair County to the Illinois House of Representatives as a member of the Democratic Party. He only served briefly in the House, where his colleagues included Abraham Lincoln and his future Senate colleague William Alexander Richardson. In 1841, Stephen A. Douglas resigned as Secretary of State of Illinois to become a member of the Illinois Supreme Court, and Governor Thomas Carlin appointed Trumbull to succeed him. Trumbull remained Secretary of State for two years, devoting most of his time to his legal practice while his brother Benjamin cared for the routine duties of the office. In 1843, Governor Thomas Ford requested Trumbull's resignation after he criticized Ford's position on the State Bank of Illinois. In 1842, the Bank had suspended payments after the value of its notes had fallen to fifty cents on the dollar, and Trumbull considered repeated efforts to legalize the suspension futile and disgraceful. Instead, and contrary to Ford's stated policy, Trumbull called for immediate liquidation of the Bank. His resignation divided the Illinois Democratic Party, with the Trumbull faction including Virgil Hickox, Samuel H. Treat, Ebenezer Peck, and Mason Brayman. Trumbull then returned to Belleville to practice law and marry Julia Jayne, a physician's daughter and a friend of Mary Todd Lincoln.