 He served during the Canada border disturbances on garrison duty and then was promoted to first lieutenant in April 1846.Ricketts saw considerable action during the Mexican–American War, participating in the Battle of Monterrey, and as commander of a gun, along with Abner Doubleday, held the Rinconada Pass during the Battle of Buena Vista.Despite his active service, he received no brevet promotions during the Mexican–American War, unlike many of his fellow officers.