Article VI, Section 19 of the Constitution of the State of Illinois creates the office of state's attorney: A State's Attorney shall be elected in each county in 1972 and every fourth year thereafter for a four-year term. One State's Attorney may be elected to serve two or more counties if the governing boards of such counties so provide and a majority of the electors of each county voting on the issue approve. A person shall not be eligible for the office of State's Attorney unless he is a United States citizen and a licensed attorney-at-law of this State. His salary shall be provided by law.
The powers and duties of the state's attorney are set forth by the Counties Code within the Illinois Compiled Statutes (55 ILCS 5/3-9005).
For the first 54 years of statehood, Illinois had no county-based prosecutor offices. Under Illinois's first two constitutions and applicable laws, prosecutors were elected or appointed in each of the state's judicial circuits which almost always included more than one county. These officers, interchangeably called circuit attorneys and state's attorneys had the duty of prosecuting all criminal cases, as well as providing legal advice to any state or county officials in their jurisdiction. Illinois' 1870 Constitution provided that in 1872 and every four years thereafter, the voters of each county would elect a state's attorney. The way in which Illinois voters have selected their local prosecutors has remained the same since.
In 1819, the first general assembly adopted legislation providing for the duties and selection of the attorney general and three circuit attorneys with the attorney general also serving as the circuit attorney in the First Circuit in downstate Illinois. These early prosecutors were appointed by the governor with the advice and consent of the Illinois Senate and were entrusted with the responsibility to prosecute "all matters and things, pleas, actions, and suits, wherein the state is a party." In 1827, the term "circuit attorney" was temporarily replaced with the term "state's attorney" when the general assembly revised the law proscribing the duties of the attorney general and local prosecutors. Section 1 of that legislation provided that it shall be the duty of the attorney general to "when required, to give his opinion without fee or reward to any county commissioner's court [county board] - "when required to do so, upon any question of law relating to any criminal or other matter in which the people, or any county, is concerned". Section 4 of the 1827 act directed state's attorneys to "do and perform the duties, within the judicial circuit in which he shall reside, which are, by the first section of this act, required of the attorney general." Accordingly, as early as 1827, state's attorneys have served in the dual role as prosecutors and counselors to county government.
In 1835, the general assembly assumed the responsibility of electing state's attorneys for terms of two years, ending the previous system of appointment by and service at the pleasure of the governor. It was during this time - 1836 - that the area that is now Kane County was incorporated.
In the 1840s, the general assembly re-designated local prosecutors as "circuit attorneys," but this change was brief. Beginning with the ratification of the Constitution of 1848, circuit-based prosecutors became state's attorneys again. Interestingly, from 1848 until 1867, Illinois functioned without an elected or appointed attorney general and the state's attorneys served as the sole prosecuting authority in the state. During this time, state's attorneys were popularly elected for four-year terms in each of the multi-county judicial circuits. Between 1848 and 1870, statutes occasionally used the term "prosecuting attorney" to refer to state's attorneys as the 1848 constitution permitted the general assembly to provide for the election of county prosecutors in place of circuit-wide elected state's attorneys.
The state's third constitution, ratified in 1870, provided that in 1872 and every four years thereafter, state's attorneys were to be elected by county, rather than by judicial circuit. Since 1870, continuing after the 1970 constitution, the principal powers and duties of the state's attorneys have remained largely the same - with some of the statutory provisions still retaining language from the 1820s.