During E.R.S. Canby's military career, Louisa joined him on assignments with the almost sole exception of the Mexican–American War. In his memoirs, William Tecumseh Sherman recalls the arrival of the Canbys at Monterey, California, in early 1849 where then-Major Canby succeeded Sherman as adjutant-general of the military Department of California. The Canbys, with their six-year-old daughter, Mary (who died in childhood), took up residence in Monterey which was then the military headquarters for California. (Benicia, California was soon added as the headquarters for the Pacific.) About this time, Louisa met Lt. Col. Henry Stanton Burton, who became involved in controversy when he proposed marriage to Maria Amparo Ruiz, the granddaughter of the former Mexican governor of Baja California. (She was a remarkable woman in her own right: widely admired for her beauty and aristocratic carriage, she later became a successful novelist.) The announcement of their engagement set off a firestorm as the Roman Catholic Bishop of California condemned the union (Burton was a Protestant), and the governor declared that "all the authorities of California are not to authorize any marriage when either of the parties is a Catholic." Louisa offered the couple the use of the Canby home where their marriage took place on July 7, 1849. Major Canby, who had begun a tour of northern California on July 2 and did not return to Monterey until August 9, was forced to explain that he had taken no part in the affaire and that his wife, a civilian, had acted alone.