In March 1862, Patrick was appointed as a brigadier general of volunteers. His brigade was assigned to the division of Brig. Gen. Rufus King in the army of Maj. Gen. Irvin McDowell and sent to the Shenandoah Valley in northern Virginia, where Patrick's men occasionally fought against Confederates under Stonewall Jackson. He was subsequently appointed military governor of Fredericksburg, Virginia, in April 1862. Transferred later in the year to the Army of the Potomac under Maj. Gen. George B. McClellan, he commanded the 2nd Brigade, 3rd Division, I Corps in the defenses of Washington, D.C.. Patrick's brigade suffered hundreds of casualties in the Maryland Campaign, seeing action at the battles of South Mountain and Antietam. In the reorganization of the command structure following Antietam, Patrick was named as the provost marshal for the Army of the Potomac on October 6, 1862. To support his efforts, he had the equivalent of a brigade of troops, including at various times:
Patrick tried vainly to stop vengeful Union soldiers from sacking and looting Fredericksburg in November, and had to fend off political officials who placed the blame on him, including numerous inquiries from Secretary of War Edwin M. Stanton. Nevertheless, Patrick continued in the role of provost marshal throughout 1863. New army commander Maj. Gen. Joseph Hooker had Patrick create the Bureau of Military Information, a network of intelligence agents. At the Battle of Gettysburg, Patrick oversaw the processing of thousands of Confederate prisoners of war. In early 1864, when Ulysses S. Grant arrived in the Eastern Theater and assumed authority over multiple armies, Patrick was elevated to provost marshal for the combined forces operating against Richmond, Virginia.
Postbellum career
Following the surrender of Robert E. Lee in April 1865, Patrick remained in northern Virginia as provost of the District of Henrico in the Department of Virginia. Although appointed a brevet major general in the volunteer army, Patrick resigned from the Army a second time on June 12, 1865, preferring to return to civilian life rather than accept a role in the smaller postbellumregular army. In 1865, he ran on the Democratic ticket for New York State Treasurer but was defeated by RepublicanJoseph Howland. Patrick moved to Manlius, NY, and from 1867 through 1868, Patrick served as president of the New York StateAgricultural Society, then spent the next two years as a state commissioner, a role he again held from 1879 through 1880. He became a widely known public speaker, particularly on topics related to technological advances in agriculture. Interested in the care of former soldiers, Patrick moved to Ohio and became the governor of the central branch of the National Home for Disabled Volunteer Soldiers. A large tombstone bearing his name and rank stands in the Manlius Village Cemetery. Marsena Patrick died in Dayton, Ohio, and was buried in the Dayton National Cemetery. His diary, frequently critical of the Army's commanders, was published in 1964.
Patrick appears briefly in the 2003 Civil War film Gods and Generals, portrayed by Ryan Cutrona. During scenes depicting the Battle of Fredericksburg, he is seen berating soldiers looting the town, and ordering his guard to put a stop to the vandalism.
