In 1805, Lucas was chosen by President Thomas Jefferson to serve as a superior judge and land commissioner in the new Louisiana Territory. Moving to St. Louis, he soon became deeply involved in a series of disputes involving land grants that had been made to St. Louisans by the Spanish prior to the Louisiana Purchase. Lucas took a hard line on land claims, insisting that all such claims should meet the requirements of Spanish law. In doing so, he alienated many of the City’s leading families, who were anxious to protect their holdings.

Lucas later was a central figure in the debate over slavery in Missouri. A slaveowner himself, Lucas nonetheless opposed slavery, on economic rather than moral grounds. When the new state of Missouri held elections for its first U.S. senators, Lucas ran for one seat against Thomas Hart Benton, the man who had killed his son Charles in a duel three years earlier. Lucas lost by a small margin.

Lucas also succeeded in acquiring a great deal of property in and around the City. He developed a 10-square-block area between Seventh and Ninth Streets from Market north to St. Charles Avenue in 1833. By 1845, he had expanded this subdivision further west to Eleventh Street. (Primm, 132-144)