Large companies sometimes preserved records about their employees. These records usually contain hiring and termination details and may include biographical data about the employees and possibly their families. If a company where your ancestor worked is still in business, it may allow limited access to its historical employee records.
Few employee records have been made public. Examples that are available for Illinois include:
Pullman Car Works (Pullman, Illinois). Employee Records, ca. 1900–1949. Salt Lake City, Utah: Genealogical Society of Utah, 1993–94. (On 68 FHL films beginning with 1908114.) (Not available at Family History Centers.) These records include employment index cards, which contain hiring and termination dates, and service forms, which contain dates and places of birth as well as information on previous employment, for about 200,000 employees who worked in the Pullman car plant.
Biographical sketches of lawyers in Illinois about 1916 can be found in Crossley’s Courts and Lawyers, cited in the "Court Records" section.
The Illinois State Archives and IRAD depositories have registers of physicians, accoucheurs, midwives, veterinarians, dentists, optometrists, nurses, surgeons, and other professions. They also have the Chicago Civil Service Commission records of policemen, firemen, and tradesmen.
Farming is one of the major occupations in Illinois. Farmers directories were published occasionally. Local farmers associations may have kept membership records with biographical information. Many farms that have been owned by the same family for 100 years or longer are mentioned in The History of Centennial Farms, cited in the "Genealogy" section.
For state employees from 1921 to 1924 and later, see the Blue Book of the State of Illinois cited in the "History" section.
Occupation records can be found in the Family History Library Catalog by using a Place Search under:
ILLINOIS, [COUNTY]- OCCUPATIONS
ILLINOIS, [COUNTY], [TOWN]- OCCUPATIONS