Marshall and his regiment were transferred to Andrew Jackson Smith's command in Missouri and took part in the pursuit of Sterling Price.

He continued in brigade command when transferred to Mobile, Alabama, to take part in the Battle of Fort Blakeley.

"[2] During William Marshall's administration, his adoptive state experienced a post-Civil-War surge of growth and development: its population doubled to 350,000, its railroad mileage quadrupled, and its commercial endeavors flourished.

After leaving office, Marshall remained active in both the private and public sectors as an attorney, banker, and as a railroad and land commissioner.

He was a partner in a law firm with Jude Kerr and Robertson Howard while residing in St. Paul, but subsequent commercial ventures faltered, as did his health.