Click here to search the Illinois State Archives for marriages
The above website says it has the complete marriage index for Johnson County from 1835-1900.
However, there might be a few missing.
If you cannot find the name you are looking for, try typing just the first letter of the Surname and click on the County. This will help in case the name is misspelled. Or try typing the name and no County (Statewide) in case they went to a neighboring County to get married.

 Marriage Licenses

 Marriage License & Certificate Images

 Marriages of Register