Officials say McDonough, who was in his mid-60s, collapsed shortly after returning from lunch. He was found by an attorney who had an appointment with him.
The lifelong Harrison resident had served on the Town Council for 17 years before becoming mayor in 1995. He was credited with spearheading the redevelopment of the 1.2-square-mile Hudson County town on the Passaic River, which has about 14,000 residents.
Town Hall was closed for the day following McDonough's death. The flag there was then lowered to half-staff.
McDonough was among a number of Democratic officials statewide who supported Republican Gov. Chris Christie for re-election last year.