Stability has been Mountainside's hallmark since it broke from neighboring Westfield to become an independent borough in 1895. Until the 1950's its population hovered around 2,000 and development was slow. But the decade that followed brought the community's greatest expansion, turning it from farmland to suburb. By the 1970's, Mountainside was almost fully developed, and in recent years only a dozen or so houses a year have been built, all on scattered lots. Mountainside continues to attract new residents, and the most recent arrivals have been young professional couples with children willing to pay the rising cost of admission.
Longtime resident say the newcomers are welcome.
Families are also drawn to the community because of its attention to education. Many youngsters attend Deerfield School, the borough's only elementary school and continue their studies at Jonathan Dayton High School in nearby Springfield, part of the Union County Regional High School District. The Superintendent at Deerfield School, Leonard J. Baccaro, said its small classes, averaging 17 students, and a veteran teaching staff promote individualized study, focusing on basic skills complemented by such courses as industrial arts and vocal music instruction, starting in kindergarten. Jonathan Dayton High School, with 739 students, offers diverse academic and business courses, ranging from social study classes on the Holocaust and Russian and Japanese cultures to instruction in Hebrew and a work-study program. The school also has graphic arts and cabinetmaking programs.