He began working for Milwaukee Road as a station agent and operator between the ages of sixteen and twenty.
In June 1907, he was appointed special counsel of Iowa, reporting to the state's attorney general, Howard Webster Byers.
Cosson won the 1908 Iowa Senate election as a Republican candidate, and represented District 17 until 1911.
Cosson contested the Republican nomination for the United States Senate seat held by Smith W. Brookhart in 1932.
In 1961, Cosson moved to his daughter's home in Des Moines, and retired from the practice of law in August 1962.