Before the war, James Williamson practiced law in Keokuk County and Des Moines, and returned to Des Moines after the war to resume his practice. He also engaged in real estate and became politically active. He was instrumental in the move of the Iowa State Capitol from Iowa City to Des Moines. His interest in western land and mining led to his appointment by President Grant as Commissioner of the General Land Office in Washington, D.C. When his interest turned to railroading, he became president of a transcontinental railroad for 10 years. He built a thriving law practice in New York, then moved to Jamestown, RI to continue that practice until his death.