Janean Watkins and Lakeesha Harris were followed by more than 100 other couples who arrived early to get licenses, the Chicago Tribune reported.
"It's historic," said Watkins, who has been with Harris for 10 years. "We wanted to be first. We wanted to make a statement. For us, for our kids. It really means something."
County clerks across the state said they were prepared for an anticipated crush of same-sex couples seeking civil union licenses Wednesday.
The Cook County Clerk's office said it would celebrate what it called "the momentous occasion" by awarding a special gift package to the first couple getting a license, the Tribune reported. In addition, several local businesses supplied gifts that will be raffled off to other couples in line.
As with marriage licenses, civil union license applicants must pay a $35 fee and wait a day before they can hold a civil union ceremony. Couples already registered as domestic partners in Cook County but want to enter into a civil union will get a $20 discount and their domestic partnership will be dissolved.
Illinois is the sixth state to allow civil unions or their equivalent, which provide couples with the same state-level rights as marriage. Hawaii and Delaware have passed civil union laws that have not yet been enacted.