Slonina provided Chicago's second, controlling a deflection in front of goal and slotting home the finish past Toronto goalkeeper Alex Bono for the 18-year-old academy defender's first goal with the Fire's first team.
Neither side struck for a goal in the second half, allowing Chicago to see out the 2-0 final.
It's the third victory of the 2020 preseason for the Fire, which moves to 3W-2L-0D in their exhibition matches with the result. The club will take the field for its final preseason match against the LA Galaxy at Dignity Sports Health Park on Saturday.
Toronto are now also 3W-2L-0D through five matches ahead of their preseason finale in Los Angeles against the Colorado Rapids on Saturday.
- CHI - Nick Slonina - 15th min.
- CHI - Elliot Collier - 31st min.
Chicago Fire FC first-half XI: Kenneth Kronholm – Brandt Bronico, Nick Slonina, Francisco Calvo, Jeremiah Gutjahr – Mauricio Pineda Djordje Mihailovic, Fabian Herbers – Elliot Collier, Przemyslaw Frankowski, Robert Beric
Chicago Fire second-half XI: Kenneth Kronholm (60' Bobby Shuttleworth) – Cristian Diaz, Johan Kappelhof, Wyatt Omsberg, Jeremiah Gutjahr (60' Jonathan Bornstein) – Micheal Azira, Alvaro Medran, Javier Casas – Allan Rodriguez, Jonathan Jimenez, Brian Gutierrez
Toronto FC first-half XI: Alex Bono; Richie Laryea, Eriq Zavaleta, Chris Mavinga, Terique Mohammed, Jonathan Osorio, Noble Okello, Alejandro Pozuelo, Jayden Nelson, Griffin Dorsey, Ayo Akinola
Toronto FC second-half XI: Alex Bono; Richie Laryea, Eriq Zavaleta, Rocco Romeo, Terique Mohammed, Adolfo Ovalle, Noble Okello, Ifunanyachi Achara, Jacob Shaffelburg, Griffin Dorsey, Ayo Akinola