[3] With the outbreak of the Civil War, Maltby enlisted as a private in the 45th Illinois Infantry (known as the "Lead Mine Regiment") on December 26, 1861.

The following year he commanded his Illinois troops in Ulysses S. Grant's operations against the Confederate defenses of Vicksburg, Mississippi.

The resulting explosion blew apart the Confederate lines, while troops from John A. Logan's division of the XVII Corps followed the blast with an infantry assault.

Maltby's 45th Illinois charged into the 40-foot (12 m) diameter, 12-foot (3.7 m) deep crater with ease, but were stopped by recovering Confederate infantry.

The Union soldiers became pinned down while the defenders rolled artillery shells with short fuses into the pit with deadly results.

[3] His brother William H. Maltby was the captain of a Confederate artillery battery and was taken as a prisoner of war in a skirmish on Mustang Island along the Texas Gulf Coast.