During the Mexican–American War, Hall enlisted as a private in the First Missouri Cavalry Regiment and later was promoted to lieutenant.

In October he accompanied the famous Mormon Battalion on its march to California, thereafter he returned to Missouri and assumed his seat in Congress.

In 1861 Hall was a member of the constitutional convention, which affirmed the policy of armed neutrality put forth by outgoing governor Robert Marcellus Stewart: that Missouri would remain in the Union but would not send troops or supplies to either side.

Union General Nathaniel Lyon seized the state capital and deposed Jackson.

After leaving his unelected office at the war's end, Hall resumed his law practice in St. Joseph.