The companies that composed the Illinois 15th Cavalry regiment were independent companies attached to infantry regiments and acted as such. In the spring of 1862 they moved with the army from Cairo up the Tennessee River to Fort Henry and later participated in the battle of Shiloh. In the spring of 1863 the companies were organized as the 15th regiment and during the following year did post duty and severe scouting. Each company saw separate service with only Company K participating in the Atlanta Campaign and the March to the Sea.
By expiration of term of service, the 15th was mustered out Aug. 25, 1864, and the recruits who were enlisted in 1862 were consolidated with the 10th Illinois cavalry.
Company A - Alexander County (Also known as Stewart's Independent Cavalry Company, Stewart's Independent Battalion Cavalry - 15th Illinois Cavalry, Companies A - F)
Company B - Massac County (Also known as Carmichael's Cavalry Company)
Company C - Franklin County (Also known as Dollins' Cavalry Company)
Company D - Clinton County (Also known as Clinton County Cavalry, O'Harnett's Cavalry Company)
Company E - Jackson County (Also known as Hutchens' Cavalry Company)
Company F - Franklin County
Company G - Kane County (Also known as Gilbert's Cavalry Company)
Company H - Kane County (Also known as Wilder's Cavalry Company, Steele's Escort, Kane County Cavalry)
Company I - Kane County and Kendall County (Also known as Jenks' Cavalry Company or Dragoons, Hamilton's Escort, Rosecrans' Escort, Willis' Cavalry Company)
Company K - Kane County (Also known as Slocum's Escort, Dodson's Cavalry Company, Hooker's Escort, Davis' Escort, Sherer's Cavalry Company Dragoons, Smith's Cavalry Company, Williams' Escort)
Company L - LaSalle County and Bureau County (Also known as Ford's Cavalry Company, Grant's Escort)
Company M - Winnebago County
