He gained fame as a defense attorney, defending notables such as Sandford of the Dred Scott case, Maj. Gen. Fitz John Porter at his courts-martial, and Mary Surratt, alleged conspirator in the assassination of Abraham Lincoln.

In 1817, Johnson moved to Baltimore,[6] where he became a legal colleague of Luther Martin, William Pinkney and Roger B. Taney, the Attorney General and later Chief Justice of the United States from 1835 until 1864.

From his confirmation by the Senate in March 1849 until July 1850, Johnson was Attorney General of the United States under President Zachary Taylor.

In November 1856, a large crowd, armed with guns and clubs, burned an effigy of Johnson on the railing of the Battle Monument in front of his house to protest a speech he made in New York against President Fillmore.

[2][11] However, Johnson was personally opposed to slavery and became a key figure in the effort to keep Maryland from seceding from the Union during the American Civil War.

The officers on the court-martial, all handpicked by Secretary of War Edwin Stanton, voted to convict Porter of cowardice and disobedience.

After the war, reflecting the diverse points of view held by his fellow statesmen, Johnson argued for a gentler Reconstruction effort than that advocated by the Radical Republicans.

[22] Soon after his arrival in England negotiated the Johnson-Clarendon Treaty for the settlement of disputes arising out of the Civil War, including the Alabama Claims.

[23] The Senate, however, refused to advise and consent to ratification, and he returned home on the accession of General Ulysses S. Grant to the presidency.

"[30] In December 1875, he wrote a letter to the Baltimore Sun discussing the potential impact of England's purchase of a controlling interest in the Suez Canal.

[31] In early 1876, Johnson was in Annapolis arguing the case of Baker v. Frick in the Court of Appeals and was a guest at the Maryland Governor's Mansion.

[35] Together, Reverdy and Mary had 15 children, of which five daughters and three sons survived, including:[36] After his death on February 10, 1876,[40] Johnson was buried at the Greenmount Cemetery in East Baltimore.