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 Governor Livingston High School in Berkeley Heights. Most of the Mountainside's social and civic organizations are associated with the school or the Borough's three churches, the oldest of which, Mountainside Union Chapel, was built in 1801. There are also such annual holiday events at a Fourth of July picnic and a Memorial Day parade to promote community spirit. The Watchung Reservation and its woodland nature trails are a major recreational resource for the borough.