His regiment fought in James G. Blunt's division in the battles of Old Fort Wayne, Cane Hill, and Prairie Grove.

Although he possessed no military experience before the civil war, Ewing was promoted to brigadier general on March 13, 1863, for his leadership at the Battle of Prairie Grove.

Ewing was responsible for General Order № 11, issued in retaliation for William Quantrill's raid on Lawrence, Kansas, where 450 raiders shot and killed 150 men and boys.

[3] The order commanded that civilians with southern sympathies living in four Missouri counties be expelled, and if they did not leave voluntarily, they would be forced out by Union cavalry.

His command of 1500 heavily outnumbered soldiers (900 effectives)[4] and a few black civilians fought off repeated attacks from a force of about 15,000 Confederates, buying additional time for the Union army to strengthen the defenses around St. Louis.

Instead of surrendering, Ewing and his men successfully eluded Price's force during the night and fought a fighting withdrawal to Rolla, Missouri.

[7] Although a staunch friend and ally of Abraham Lincoln, when Edwin Stanton engaged in a post-assassination flap with Ewing's brother-in-law William T. Sherman over final surrender terms to the Southern armies, Ewing agreed to represent two of John Ford's (of Ford's theater) employees in the Lincoln conspiracy trials.

For their roles in the assassination, Mudd, Arnold and Spangler were sentenced to federal prison at Fort Jefferson in the Dry Tortugas off Key West, Florida.

In 1870, he returned to his native Lancaster, Ohio, where he practiced for the next decade and attempted several business investments in railroads and telegraph companies.

There is scholarly controversy whether the use of George Caleb Bingham's painting General Order № 11 in the anti-Ewing campaign made the difference in the election.

Ewing made a notable address before the Marietta Centennial Convention of 1887, and one before the Kansas state bar association in 1890.