Mountainside is a borough in Union County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 6,685. Mountainside was incorporated as a borough in 1895. New Jersey Transit provides bus service to the Port Authority Bus Terminal in Midtown Manhattan Newark Liberty International Airport is about 10 miles away. Convenient location offers highways in and around Mountainside that include Route 22, 78 and 28. Students in the public school system attend pre-K through 8th grade. High school students, grades 9- 12, attend Governor Livingston High School in nearby Berkeley Heights. (1 comments)