constitutional lawyer, U.S. senator from Maryland (1845–49, 1863–68), attorney general under President Zachary Taylor (1849–50), and minister to Great Britain (1868–69). Able to grasp either side of an issue, he was called “the Trimmer” for his ability to bring about compromises.

Although he lived in a slave state, Johnson opposed slavery and the annexation of possible slave territory after the Mexican War (1846–48). He helped to keep Maryland in the Union during the American Civil War (1861–65). Nonetheless, he represented the slave-owning defendant in the famous Dred Scott (Dred Scott decision) case (1857), in which the U.S. Supreme Court decided that blacks could not be citizens of the United States.

During the Civil War Johnson generally opposed extreme measures, and afterward he sought a moderate reconstruction program for the defeated South. As a senator he voted for a wartime loyalty oath, but after the war he was instrumental in having test oaths declared unconstitutional. He aided in securing acquittal in impeachment proceedings against President Andrew Johnson (1868). As minister to Great Britain he prepared the way for the peaceful settlement of disputes between that nation and the United States over British policy toward the Confederacy.