Mountainside is well-named, for the borough lies along the southeast face of the Watchung Mountains. In 1895, its citizens, including those from the hamlets of Branch Mills, Baltusrol and Locust Grove, formed the Borough of Mountainside. The area previously had been a rural, undeveloped portion of Westfield.

Several Mountainside farmhouses date

from the colonial period through the early nineteenth century. They include the Deacon Andrew Hetfield House, now the headquarters and museum of the Mountainside Historic Preservation Committee. The early industries in Mountainside were the mills on Nomahegan Creek. A gristmill, established in 1810, was purchased by Charles and Aaron Parkhurst in 1851. They established Branch Mills, a manufacturer of pasteboard. The mills, located near the Springfield Avenue entrance to Echo Lake Park, were closed and demolished in the early 20th century with most of the land becoming parkland.

Suburban growth following World War II caused the population to double by 1950 and quadruple by 1970. Despite growth, Mountainside has vast open space due to Echo Lake Park and the Watchung Reservation, each part of the Union County Park System. The Reservation is home to the Watchung Stables and the Trailside Nature and Science Center. Mountainside’s small but pleasant business district is located on Mountain Avenue near New Providence Road.

Mountainside is home to Children's Specialized Hospital, dating back to the 1890's when the property was purchased from the John Drew Estate. The original home remains a focal point on the sprawling complex.