On March 12, 1864, Grant was appointed general-in-chief of the United States Army and moved his headquarters to the eastern theater in Virginia.
 Grant asked Comstock to come with him as his senior aide-de-camp.
 Grant determined to command in the field, following and directing the movements of the Army of the Potomac in a major offensive during the summer of 1864 known as the Overland Campaign.
 During the campaign, Comstock played a key role in coordinating the movements of the various corps of the army and personally conveying Grant's orders to the corps commanders.
 His efforts were particularly successful during the Battle of the Wilderness for which Comstock won a commendation from Grant and a brevet promotion to lieutenant colonel in the Regular Army.

Comstock was temporarily detached from Grant's staff and appointed by Grant to the post of chief engineer of the Department of North Carolina in January 1865.
 The transfer was prompted by Maj.
Gen. Benjamin Butler's failure to take Fort Fisher in December 1864.
 The fort was the last Confederate stronghold on the east coast.
 Serving under the command of Maj.
Gen. Alfred Terry, Comstock assisted in planning a second and successful assault on Fort Fisher.
 Following this victory, Comstock was awarded the grade of brevet colonel in the Regular Army.
 Comstock also was nominated by President Abraham Lincoln on January 23, 1865 for the award of the honorary grade of brevet brigadier general, U.S. Volunteers, to rank from January 15, 1865, for gallant services in the capture of Fort Fisher and the U.S. Senate confirmed the award on February 14, 1865.
He returned to Virginia and to his role as Grant's senior aide, but did not remain long.

In March 1865, Grant again dispatched Comstock to aid in a key siege operation—this time to Mobile, Alabama, the last Confederate stronghold on the Gulf of Mexico.
 There Comstock served under Maj.
Gen. Edward Canby and assisted in operations leading to the surrender of Mobile after the Battle of Spanish Fort and the Battle of Fort Blakely in April 1865.
 While Comstock was serving in Alabama, Gen. Robert E. Lee surrendered to Grant after the Battle of Appomattox Courthouse, essentially ending the Civil War.
 On January 13, 1866, President Andrew Johnson nominated Comstock for the award of the honorary grade of brevet major general, U.S. Volunteers, to rank from March 26, 1865, for faithful and meritorious services during the campaign against the city of Mobile and its defenses, and the U.S. Senate confirmed the award on March 12, 1866.
On April 10, 1866, President Andrew Johnson nominated Comstock for the award of the honorary grade of brevet brigadier general, U.S. Army, (Regular Army), to rank from March 13, 1865, for gallant and meritorious services in the campaign ending with the capture of Mobile, Alabama, and the U.S. Senate confirmed the award on May 4, 1866.
On March 12, 1864, Grant was appointed general-in-chief of the United States Army and moved his headquarters to the eastern theater in Virginia.  Grant asked Comstock to come with him as his senior aide-de-camp.  Grant determined to command in the field, following and directing the movements of the Army of the Potomac in a major offensive during the summer of 1864 known as the Overland Campaign.  During the campaign, Comstock played a key role in coordinating the movements of the various corps of the army and personally conveying Grant's orders to the corps commanders.  His efforts were particularly successful during the Battle of the Wilderness for which Comstock won a commendation from Grant and a brevet promotion to lieutenant colonel in the Regular Army.
Comstock was temporarily detached from Grant's staff and appointed by Grant to the post of chief engineer of the Department of North Carolina in January 1865.  The transfer was prompted by Maj. Gen. Benjamin Butler's failure to take Fort Fisher in December 1864.  The fort was the last Confederate stronghold on the east coast.  Serving under the command of Maj. Gen. Alfred Terry, Comstock assisted in planning a second and successful assault on Fort Fisher.  Following this victory, Comstock was awarded the grade of brevet colonel in the Regular Army.  Comstock also was nominated by President Abraham Lincoln on January 23, 1865 for the award of the honorary grade of brevet brigadier general, U.S. Volunteers, to rank from January 15, 1865, for gallant services in the capture of Fort Fisher and the U.S. Senate confirmed the award on February 14, 1865. He returned to Virginia and to his role as Grant's senior aide, but did not remain long.
In March 1865, Grant again dispatched Comstock to aid in a key siege operation—this time to Mobile, Alabama, the last Confederate stronghold on the Gulf of Mexico.  There Comstock served under Maj. Gen. Edward Canby and assisted in operations leading to the surrender of Mobile after the Battle of Spanish Fort and the Battle of Fort Blakely in April 1865.  While Comstock was serving in Alabama, Gen. Robert E. Lee surrendered to Grant after the Battle of Appomattox Courthouse, essentially ending the Civil War.  On January 13, 1866, President Andrew Johnson nominated Comstock for the award of the honorary grade of brevet major general, U.S. Volunteers, to rank from March 26, 1865, for faithful and meritorious services during the campaign against the city of Mobile and its defenses, and the U.S. Senate confirmed the award on March 12, 1866. On April 10, 1866, President Andrew Johnson nominated Comstock for the award of the honorary grade of brevet brigadier general, U.S. Army, (Regular Army), to rank from March 13, 1865, for gallant and meritorious services in the campaign ending with the capture of Mobile, Alabama, and the U.S. Senate confirmed the award on May 4, 1866.
Cyrus B. Comstock