He entered the Mexican–American War as a first lieutenant and was cited for bravery and noteworthy service at the Battles of Contreras and Churubusco.

He stayed with his brigade on the Peninsula afterwards, and spent the rest of the war holding various commands in the Virginia-North Carolina area.

[1] Palmer successfully defended a garrison at New Bern, North Carolina from a Confederate attack led by Maj. Gen George Pickett on February 1, 1864 and his command linked up with William T. Sherman's army as it drove up through the state in March 1865.

He died on the morning of September 10, 1900 of complications from kidney failure in Chevy Chase, Maryland.

His son in law was Major General Eben Swift who at one time commanded the 5th Cavalry and his grandson and namesake was Major General Innis Palmer Swift, who commanded the 1st Cavalry Division and later the I Corps in the South Pacific in World War II.