In Washington, the regiment was assigned to construct fortifications and maintain a picket until March 1862, when they commenced a march with General George B. McClellan's Peninsula campaign. Captain Clark, however, left the regiment in early April 1862, as he was appointed adjutant on the staff of his brother-in-law Brigadier General Joseph B. Plummer in the Army of the Mississippi. He arrived in time to participate at the Siege of Corinth. General Plummer had earlier been wounded in battle, and died of his wounds and prolonged exposure in August. On Plummer's death, General William Rosecrans, commanding the Army of the Mississippi, took Captain Clark onto his staff.