The ruling reverses decisions by the lower state courts. Chicago is the county's seat.
Gun rights advocates had argued the ban "violates the due process and equal protection clauses of the United States Constitution and violates the Second Amendment right to bear arms," Justice Mary Jane Theis said in her opinion. They also argued that the ban is too vague.
Under the ordinance ban, anyone "who prior to the enactment lawfully possessed assault weapons or large capacity magazines had 90 days from the effective date to surrender the weapons to the sheriff, to remove the weapons from the county or to modify the weapons to render them inoperable or no longer defined as an assault weapon."
Violations could bring a jail sentence of six months and a fine between $500 and $1,000.
A lower state appeals court had ruled that the Supreme Court's 2008 decision in District of Columbia vs. Heller striking down handgun restrictions in the federal capital did not apply to the states.
But Theis said the Illinois justices cannot "say conclusively at this early stage of the litigation that assault weapons as defined in this ordinance categorically fall outside the scope of the rights protected by the Second Amendment."
She also said that unlike the parties in the U.S. Supreme Court case, Cook County "has not had an opportunity to present evidence to justify the nexus between the ordinance and the governmental interest it seeks to protect. Pursuant [to state law] we cannot say at this point that it is clearly apparent that no set of facts can be proved that would entitle [the challengers] to relief on count IV [the constitutional issues]. ... Accordingly, for these reasons, we reverse the trial court's dismissal of the first amended complaint with respect to count IV and remand to the trial court for further proceedings."
In other words, the challengers are now allowed to present their argument at the trial court level.