Moving with his family to Illinois at a young age, Gillespie fought in the Black Hawk War before studying law at Transylvania University.
Upon graduation, he was elected to a two-year term in the Illinois House of Representatives, where he once jumped out of a window with Abraham Lincoln to stop a quorum.
[1] In 1839, the Whig representatives learned of an attempt by House Democrats to suspend the Whig-controlled state banks.
Democrats managed to bring enough members to take the quorum and locked the doors to keep the Whigs in session.
In 1850, Gillespie worked with two other senators to force the Illinois Central Railroad to pay 7% of its gross earnings to the state.
[1] After his final Senate term expired, he returned to his law practice, partnering with his nephew David Gillespie.
In 1856, Gillespie was selected as one of the ten delegates to the Bloomington Convention, the founding of the Illinois Republican Party.