As a young man, Mosby read law and was admitted to the bar in 1846. He served as a volunteer in the Mexican–American War with the rank of captain in Colonel Alexander W. Doniphan's regiment and was cited for gallantry at the Battle of Sacramento on February 28, 1847.
In his early years, Mosby studied law and obtained bar admission in 1846. He served as a volunteer with the rank of captain under Colonel Alexander W. Doniphan's regiment during the Mexican–American War and was honored for bravery at the Battle of Sacramento on February 28, 1847.