His mother, Nancy Grist, was the daughter of a prominent general from Georgia; she died when Grimes was only four months old, and his older sister for a time raised him.

He resigned from the commission after the passage of the Ordinance of Secession and joined the Confederate Army as the major of the newly formed 4th North Carolina Infantry on May 16, 1861.

Grimes was promoted to lieutenant colonel on May 1, 1862, and fought at the Battle of Seven Pines, during which he was wounded when his injured horse fell on top of him on May 31.

[3] On June 19, 1862, Grimes was promoted to the rank of colonel and given command of the 4th North Carolina Infantry, now part of the Army of Northern Virginia.

Upon recovery Grimes returned to field duty in temporary command of an infantry brigade within the division of Maj. Gen. Daniel Harvey Hill.

During the 1864 Overland Campaign, Grimes was promoted to the rank of brigadier general on May 19, and given permanent command of his brigade of North Carolinians.

When Maj. Gen. Stephen D. Ramseur was killed at Cedar Creek, Grimes assumed command of his division on December 9 and led it for the rest of the war.

[4] He served in the trenches surrounding Petersburg and joined Robert E. Lee's retreat to the west that ended when the way was blocked by Federal columns near Appomattox Court House.

Grimes led an attack that temporarily cleared Federals from the Lynchburg Road, briefly opening up a possible route of escape for a portion of Lee's army.

In 1880, Grimes was ambushed and killed in Pitt County, North Carolina, by a hired assassin named William Parker, presumably to prevent him from testifying at a criminal trial.

Friends and family erected a cenotaph at Trinity Episcopal Cemetery in Chocowinity, North Carolina, with his accomplishments and Civil War service inscribed upon it.