40 ILCS 5/8-237

Whenever territory is annexed to the city, any person then employed as a municipal employee in the annexed territory, who shall be employed by the city 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 city. Such employee shall be treated, as of the date such annexation comes into effect the same as is a present employee of the city on the effective date.