The majority of Symmes H. Stillwell's Civil War papers is correspondence, which consists almost exclusively of letters from Stillwell to his mother, Hannah Stillwell. There are also a couple of letters from Stillwell to his brother, Daniel P. Stillwell, and his cousin, Peter R. Bergen. Other materials in the series include Stillwell's diaries that he kept during his service; documents; poems and song ballads written by Stillwell and others; and other Civil War-related materials, such as papers regarding Stillwell's war pension and ephemera relating to the Roanoke Associates, an organization for veterans of the Battle of Roanoke Island.

The papers in this series document Stillwell's experience and also provide information about the 9th New Jersey Volunteers. The letters, in particular, provide detailed descriptions and offer insight into various aspects of the war, such as camp life, battles, the aftermath of battles, widespread sickness amongst the troops, and the significant amount of travel undertaken by the 9th on land and on sea. They also reveal Stillwell's political leanings, his enthusiasm for the war (at least initially), his confidence in battle, and his opinions about African Americans, specifically black military regiments. For example, shortly after the Emancipation Proclamation was issued, Stillwell wrote to his mother: "...for my part, I consider the arming of negroes a confession of weakness, a folly, an insult to the brave Soldier, and a crime against humanity and civilization...I hope the time will come when the President will have to recall his proclamation."(March 8, 1863)