Powell, Mary Surratt, Herold, and George Atzerodt were sentenced to death by a military tribunal, and executed at the Washington Arsenal.

West of the Atlantic Fall Line the central and western parts of the state largely being settled by Germans from Pennsylvania and other Northerners during the 18th Century, tended to support the Union, as evidenced by Governor Thomas Holliday Hicks' plan to stop secession by moving the Maryland General Assembly to Frederick, where on April 29, 1861, secession had been voted down 53–13, this taking place before Maryland was placed under martial law.

The few exceptions to this rule included Hicks' successor, Augustus Bradford, who had his home in North Baltimore burned to the ground in 1864 by Frederick native Confederate General Bradley T Johnson and his Maryland troops.

Johnson was the first to concoct the idea of kidnapping President Lincoln while he was visiting the soldiers home, but whether there were any direct connections to Booth and Powell is unknown.

[20] He crossed into Virginia and on September 30, he ended up at the home of John Scott Payne, a prominent doctor and Confederate sympathizer who lived at Granville Tract, a plantation about 4 miles (6.4 km) from Warrenton.

[24] Several other historians claim that the Confederate Secret Service had already recruited him into its ranks during the previous year—with Mosby's consent—and that Powell's moodiness came from moral misgivings he had as he contemplated being sent north to assist with various kidnap plots against Abraham Lincoln.

[31] On January 13, he entered the Union Army's lines at Alexandria, claimed to be a civilian refugee,[31] and—under the name "Lewis Payne"—took an oath of allegiance to the United States.

[35][36] He used the name "Lewis Payne", and the Bransons introduced him to David Preston Parr, a merchant whose china shop was used for meetings, as a mail drop, and as a safe house for Confederate agents and spies.

[38][31] The same day that Powell arrived in Baltimore, John Surratt and Louis J. Weichmann purchased a boat at Port Tobacco in Charles County, Maryland.

Surratt, and to a much lesser degree Weichmann, were members of a group led by John Wilkes Booth which planned to kidnap President Abraham Lincoln and spirit him into Virginia, where he could be turned over to Confederate military authorities.

Booth had assembled his entire team—which consisted of John Surratt, Lewis Powell, Samuel Arnold, George Atzerodt, David Herold, and Michael O'Laughlen—because he wanted the men to plan to kidnap Lincoln the next time he attended a play at Ford's Theatre.

[54][55] On the morning of March 17, Booth learned that President Lincoln had been invited to attend a matinee theatrical performance at the Soldiers' Home.

The Soldiers' Home was in a rural part of the District of Columbia about 1 mile (1.6 km) from the city limits (at that time Florida Avenue), and Lincoln usually visited the facility without an escort.

Booth sent Herold out to the Surratt tavern with equipment, and informed the others that they should wait at a local saloon while he rode out to the Soldier's Home to scout the area.

[60] On March 23, Booth sent a coded telegram to Louis Weichmann, which John Surratt understood to mean that Powell should stay at the Herndon House after he returned to Washington.

[63] Powell joined Booth that night to watch a performance of the opera La Forza del Destino at Ford's Theatre.

There is testimony from the nurse attending the Secretary of State indicating that Powell may have learned of his role to assassinate Seward on Thursday, April 13.

At about 10:10 p.m., the same time Booth made his way to the unguarded presidential box at Ford's Theater, Powell was escorted to the Seward residence on Lafayette Square near the White House by David Herold.

Powell nonetheless managed to cut through Seward's right cheek and along the right side of his throat, causing a large amount of blood flow.

It is well-established that he ended up (by riding or walking) in the far northeast part of the District of Columbia near Fort Bunker Hill, where he discarded his overcoat.

[h][76] In the overcoat pockets were Powell's riding gloves, a false mustache, and a piece of paper with Mary Gardner's name and hotel room number on it.

[88] During much of the Victorian era, it was considered unseemly for any man (even a menial laborer) to be seen in public without a hat, and Powell would have been viewed with suspicion had he tried to enter the city without one.

[99] Taken into custody, Powell was discovered to have a box of pistol cartridges, a compass, hair pomade, a brush and comb, two fine handkerchiefs, and a copy of his oath of allegiance (signed "L. Paine") in his pockets.

Powell was formally arrested, and imprisoned aboard the USS Saugus, a Union monitor then at anchor in the Anacostia River at the Washington Navy Yard.

[101] A second identification was made around mid-morning on April 18 when Augustus Seward visited the Saugus and positively identified Powell as the man who attacked him and his father.

Powell obliged Gardner by posing seated, standing, with and without restraints, and modeling the overcoat and hat that he had worn the night of the Seward attacks.

Among the most famous of the photographs is one in which Powell sits against the gun turret of the Saugus, staring into the camera in a modern fashion, relaxed and direct.

[115] Living conditions improved again on June 18, when the prisoners were given a box to sit on, outdoor exercise time each day, and reading material and chewing tobacco after each meal.

[145] A long list of other witnesses testified about minor pieces of evidence – such as the discovery of Powell's knife in the gutter and the recovery of his abandoned horse.

Powell expressed remorse for hurting Frederick Seward, but most of his discussion was disjointed and rambling, and he still could not remember his age or place of birth.