In 1814, the Illinois territorial legislature established the independent office of county treasurer, removing the treasurers’ duties from the county sheriffs. The county treasurers were required to receive the county taxes from the sheriffs, who served as collectors; to protect the county funds; to pay out county funds only on orders drawn by the county commissioners’ courts; to keep regular books of account; and to report accounts annually to the county commissioners’ courts.¹ County treasurers were appointed by the governor of Illinois Territory and served indefinite terms.

In 1819, upon the admission of Illinois to statehood, responsibility for the appointment of county treasurers was transferred to the county commissioners’ courts and the treasurers’ terms were fixed at one year. The county treasurers also gained the supplementary duty of county assessors. They were directed by the General Assembly to assess three types of property: farm land, bank stock, and "slaves or indentured servants of color. "²

The county treasurer’s office was abolished from 1825 to 1827, with the sheriffs acting as treasurers and assessments completed by independent county assessors.³ From 1827-1837 the county treasurers were appointed as before; in 1837 the county treasurers’ offices became elective, with four-year terms. That year the treasurers also became special treasurers for the county road funds.⁴ From 1839-1843, the county treasurers’ and assessors offices were again split, and in 1845, the treasurers’ terms were shortened to two years.⁵

In township-organized counties in 1849, assessment duties were transferred to the townships; in 1853, the county treasurers became ex-officio county collectors, with responsibility for supervising the township collectors, collecting taxes from non-resident county propertyholders, and overseeing the sale for taxes of the property of delinquent county taxpayers.⁶ In non-township organized counties, the treasurers remained the county assessors and the sheriffs remained the county collectors.

The county treasurer’s office became constitutional in 1870.⁷ From 1872 to 1873, non-township counties had independent county assessors; in 1873, the county treasurers took on ex-officio assessment duties again.⁸ In 1880, the county treasurers’ terms were changed to four years.⁹ In 1885, the county treasurers became ex-officio collectors and treasurers for all drainage districts larger than two townships in size.¹⁰

In 1898, the county treasurers were made ex-officio supervisors of assessments in township counties of less than 125,000 population; they had the duties of keeping the assessment books, overseeing the township assessors, revising the assessments as necessary, and publishing them.¹¹ These ex-officio assessment duties were extended to county treasurers of counties between 125,000 and 150,000 in population in 1939.¹² Between 1953 and 1969, counties were given the choice of continuing the county treasurers assessment duties, or hiring an independent county assessor; in 1969, counties were required to hire an independent county assessor by law, and the county treasurers lost this function.¹³ In 1971, the county treasurers were required to report to the county boards monthly, and in that same year the duties of ex-officio county collectors in non-township-organized counties were transferred from the county sheriffs to the county treasurers, giving that office the function of ex-officio county collector in all 102 Illinois counties.¹⁴