10 ILCS 5/25-11
Except as otherwise provided in this paragraph, when a vacancy occurs in any elective county office, or in a county of less than 3,000,000 population in the office of clerk of the circuit court, in a county which is not a home rule unit, the county board or board of county commissioners shall declare that such vacancy exists and notification thereof shall be given to the county central committee or the appropriate county board or board of county commissioners district committee of each established political party within 3 days of the occurrence of the vacancy. The vacancy shall be filled within 60 days by appointment of the chair of the county board or board of county commissioners with the advice and consent of the county board or board of county commissioners. In counties other than Champaign County operating under the county executive form of government under Division 2-5 of the Counties Code, when a vacancy occurs in an elected county office other than in the office of an elected member of the county board, the county executive shall declare that such vacancy exists and then notification of the vacancy shall be given to the county central committee of each established political party within 3 days of the occurrence of the vacancy, and the vacancy shall be filled within 60 days by appointment of the county executive with the advice and consent of the county board. However, when a vacancy occurs in the office of an elected member of the county board in a county other than Champaign County that is operating under the county executive form of government under Division 2-5 of the Counties Code, the elected county board speaker or county board chair, as the case may be, shall declare that such vacancy exists and then notification shall be given to the appropriate county board district committee of each established political party within 3 days of the occurrence of the vacancy, and the vacancy shall be filled within 60 days by appointment of the elected county board speaker or county board chair, as the case may be, with the advice and consent of the county board. In Champaign County while operating under the county executive form of government under Division 2-5 of the Counties Code, when a vacancy occurs in an elected county office or in the office of an elected member of the county board, the elected county board speaker or county board chair, as the case may be, shall declare that such vacancy exists and then notification shall be given to the county central committee or the appropriate county board district committee of each established political party within 3 days of the occurrence of the vacancy; and the vacancy shall be filled within 60 days by appointment of the elected county board speaker or county board chair, as the case may be, with the advice and consent of the county board. In counties in which forest preserve district commissioners are elected by districts and are not also members of the county board, however, vacancies in the office of forest preserve district commissioner shall be filled within 60 days by appointment of the president of the forest preserve district board of commissioners with the advice and consent of the forest preserve district board of commissioners. In counties in which the forest preserve district president is not also a member of the county board, vacancies in the office of forest preserve district president shall be filled within 60 days by the forest preserve district board of commissioners by appointing one of the commissioners to serve as president. The appointee shall be a member of the same political party as the person he succeeds was at the time of his election and shall be otherwise eligible to serve. The appointee shall serve the remainder of the unexpired term. However, if more than 28 months remain in the term, the appointment shall be until the next general election at which time the vacated office shall be filled by election for the remainder of the term. In the case of a vacancy in a seat on a county board or board of county commissioners which has been divided into districts under Section 2-3003 or 2-4006.5 of the Counties Code, the appointee must also be a resident of the county board or county commission district. If a county commissioner ceases to reside in the district that he or she represents, a vacancy in that office exists.
Except as otherwise provided by county ordinance or by law, in any county which is a home rule unit, vacancies in elective county offices, other than the office of chief executive officer, and vacancies in the office of clerk of the circuit court in a county of less than 3,000,000 population, shall be filled by the county board or board of county commissioners.
10 ILCS 5/25-11