[21] Mountainside was incorporated as a borough on September 25, 1895, from portions of Westfield Township, based on the results of a referendum held the previous day.

[1][2] Mountainside is bordered by the Union County municipalities of Summit to the north, by Springfield Township to the east, by Westfield to the south and by Berkeley Heights and Scotch Plains to the west.

Both the mayor and Borough Council are elected at-large, that is, to represent the entire community.

[3][7][37] As of 2024[update], the mayor of Mountainside is Republican Paul N. Mirabelli, whose term of office ends on December 31, 2027.

Members of the Borough Council are Deanna Andre (R, 2026), René Dierkes (R, 2025), Steven Matejek (R, 2026), Robert W. Messler (R, 2024), Donna Pacifico (R, 2025), and Donald Rinaldo Jr. (R, 2024).

[44][45][46] For the 119th United States Congress, New Jersey's 7th congressional district is represented by Thomas Kean Jr. (R, Westfield).

[48] For the 2024-2025 session, the 21st legislative district of the New Jersey Legislature is represented in the State Senate by Jon Bramnick (R, Westfield) and in the General Assembly by Michele Matsikoudis (R, New Providence) and Nancy Munoz (R, Summit).

[50] As of 2025[update], Union County's County Commissioners are: Rebecca Williams (D, Plainfield, 2025),[51] Joesph Bodek (D, Linden, 2026),[52] James E. Baker Jr. (D, Rahway, 2027),[53] Michele Delisfort (D, Union Township, 2026),[54] Sergio Granados (D, Elizabeth, 2025),[55] Bette Jane Kowalski (D, Cranford, 2025),[56] Vice Chair Lourdes M. Leon (D, Elizabeth, 2026),[57] Alexander Mirabella (D, Fanwood, 2027)[58] and Chair Kimberly Palmieri-Mouded (D, Westfield, 2027).

[59][60] Constitutional officers elected on a countywide basis are: Clerk Joanne Rajoppi (D, Union Township, 2025),[61][62] Sheriff Peter Corvelli (D, Kenilworth, 2026)[63][64] and Surrogate Christopher E. Hudak (D, Clark, 2027).

[67] Among the borough's 2010 Census population, 71.0% (vs. 53.3% in Union County) were registered to vote, including 92.5% of those ages 18 and over (vs. 70.6% countywide).

[90] Since 2006, weekday daytime coverage has been provided by Atlantic Ambulance EMS, which has a paramedic unit stationed in Mountainside.

NJ Transit Bus Operations routes 114 and 117 provide service to the Port Authority Bus Terminal in Midtown Manhattan in New York City while routes 65 and 66 travel to Downtown Newark, all making local stops at points in proximate communities.

Unlike most municipalities along Route 22, zoning in Mountainside does not allow for much commercial development adjacent to the freeway.

[95] For a one-mile stretch in the town, the eastbound shoulder on the arterial road can be used for peak hour buses.

[96] In 2012, the New Jersey Department of Transportation (NJDOT) regulated the BBS as an exclusive bus lane from 6 to 7:30am.

[97] In 2009, NJDOT funded construction of two bus turnouts along the road in nearby Union.

[98] Newark Liberty International Airport is approximately 11 miles (18 km) east of Mountainside.