Next, Rost moved to Natchitoches, Louisiana, where the majority of the people spoke French, a factor he was confident would bolster the success of his law practice.
Shortly after losing his bid for Congress, near the end of 1830 he moved with his family to New Orleans (his wife already had one or two children from a previous marriage) and resumed practicing law.
Rost continued in his law practice for the next eight years, until the summer of 1838 when he took his family on a trip to Europe to visit his mother and sisters.
Before death, he relocated his family to the newly formed Jefferson City, Missouri In 1830 Rost married Louise Odile Destrehan [des Tours] in New Orleans, becoming her second husband.
Then, he went on to marry (on Oct. 19, 1826) Marie Eleonore "Zelia" Destrehan [des Tours] (1800–1830) who was a daughter of the prior owners and a sister of Rost's wife.
Upon his return from a trip to Europe, Rost was appointed judge of the Supreme Court of Louisiana, served from March to June 1839, then resigned to engage in agricultural pursuits.