40 ILCS 5/9-224

Whenever territory is annexed to the county, any person then employed as a county employee in the annexed territory, who shall be employed by the county on the date of the annexation shall automatically come under this Article, and any service rendered for the annexed territory shall be considered, for the purpose of this Article, as service rendered to the county. Such employee shall be treated, as of the date such annexation comes into effect, as a present employee of the county on the effective date.