Griffin was promoted to captain in April 1861 and led the "West Point Battery" (officially designated as
Battery D, 5th U.S. Artillery) at the
First Battle of Bull Run in July. He received a brief furlough from the army and married Sallie Carroll, the scion of a prominent
Maryland family, on December 10. Griffin commanded his battery during the early part of the 1862
Peninsula Campaign but the unit was not heavily engaged during this time. He was promoted to
brigadier general of volunteers on June 12, 1862, to rank from June 9 and transferred to the infantry where he got command of a brigade in the V Corps. He served with distinction at
Gaines's Mill and
Malvern Hill. During the
Second Battle of Bull Run, his brigade was held in reserve. His men were also lightly engaged at the
Battle of Antietam. Griffin's irascibility frequently led to conflict with his superiors, but his leadership abilities brought steady promotion. One famous example of Griffin's hot temper came during the
Battle of the Wilderness when he was angered that his division had been driven back in disorder by a Confederate counterattack, which he said was the fault of Major General
Gouverneur Warren, the commander of the V Corps, and Major General
Horatio Wright, who commanded one of the
VI Corps's divisions, for failing to support them properly. Commanding general
Ulysses Grant's assistant adjutant general Captain
John A. Rawlins accused Griffin of inappropriate and un-soldierly language. Grant, who did not know Griffin, asked Army of the Potomac commander Major General
George Meade "Who is this General Gregg? You ought to arrest him." Meade replied "His name is Griffin and it's just his manner of speaking." Assigned command of a
division in the
V Corps, he served at the
Battle of Fredericksburg where his command lost heavily in the failed assault on Marye's Heights and during the
Chancellorsville Campaign, where they were largely held in reserve. Stricken with illness, he turned over command of the division to a subordinate and did not accompany it during the early part of the
Gettysburg campaign. Arriving as the
Battle of Gettysburg was winding down, his return was widely celebrated by his men. The popular officer led the division during the
Bristoe Campaign, but was absent during the
Mine Run Campaign when
Joseph J. Bartlett commanded the division. Griffin participated in most of the major battles of the
Army of the Potomac in 1864, including the
Overland Campaign and
Siege of Petersburg. On December 12, 1864,
President Abraham Lincoln nominated Griffin for appointment as a
brevet major general of volunteers, to rank from August 1, 1864, and the
United States Senate confirmed the appointment on February 14, 1865. Griffin assumed command of V Corps during its final campaign and was present when
Robert E. Lee surrendered at
Appomattox Court House. In August, he was assigned command of the district of
Maine, with his headquarters in
Portland. On July 12, 1865, Griffin was appointed major general of volunteers to rank from April 2, 1865. On July 17, 1866,
President Andrew Johnson nominated Griffin for appointment as a
brevet major general in the
regular army, to rank from March 13, 1865, for his service at the
Battle of Five Forks and the
United States Senate confirmed the appointment on July 23, 1866. ==Postwar==