She won in a record time of 28 minutes 41 seconds in the 8 kilometre race at the Sport Ireland Campus.
McCormack's sister Una Britton took bronze in the same race - they both next head to Lisbon for the European Championships.
This comes after she secured her place at the Toyko Olympics in 2020
McCormack ran a new personal best time in the Chicago Marathon.
Fionnuala came fifth overall in a time of 2 hours 26 minutes and 47 seconds, knocking four minutes of her previous personal best, to go 2nd in the all-time list of Irish marathon runners, behind Catriona McKeirnan.
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