Reviewed and updated October 9, 2026 by the SearchSystems.net editorial team. The arrest counts are the ones each agency reported to the FBI, as the FBI held them on October 8, 2026. The jail figures are from the Vera Institute of Justice's Incarceration Trends data. Every office link was opened and checked against the office's own site between September 23, 2026 and October 4, 2026.
Mountainside, New Jersey, in Union County, has about 7,200 residents (2024 Census Bureau estimate). A person kept in custody after an arrest in Mountainside is held at the county jail, run by the Union County Sheriff's Office, and the jail's record of each person held is a public record. This guide says where to look for someone in custody, what the jail holds and who books people into it.
This guide covers the search, the jail, who books people, state prison and common questions.
Related guides: Mountainside public records, Union County inmate search, Mountainside arrest records, Mountainside warrants, Mountainside criminal records, Mountainside court records, New Jersey arrest records and New Jersey inmate search.
Finding Someone in Custody in Mountainside
The county jail, run by the Union County Sheriff's Office, holds everyone kept in custody after an arrest in Mountainside, from every agency.
SearchSystems opened and checked the custody page of the Union County Sheriff's Office at unioncountynj.org (page title: Union County, NJ, Sheriff & Jail Info, Inmate Search), a site that blocks automated checks (HTTP 403), so open it in a browser.
Someone booked recently shows on that page; someone cited and released at the scene, or held a few hours at a city station, does not.
The Union County Jail by the Numbers
On an average day in 2020 the jail held about 330 people, down from about 420 in 2019. Its rated capacity in 2019, the last year with a capacity figure, was 995 beds, and it ran at 42 percent of it that year. The figures are from the Vera Institute of Justice's Incarceration Trends data.
Union County has about 594,000 residents (2024 Census Bureau estimate), so the jail holds about 6 people for every 10,000 residents.
Who Books People Into the Union County Jail
The Union County Sheriff's Office and the agency that reports for Mountainside (filed by the FBI as Mountainside Police Department) make the arrests that end in a booking in and around Mountainside.
The FBI's agency list also places one campus police department, two other agencies and one state police unit in Union County (the arrest records guide names each one).
The largest arrest counts reported to the FBI were Elizabeth's 2,197 for 2025, Plainfield's 1,226 for 2024 and Linden's 985 for 2023.
Together, the 20 agencies in Union County with all twelve months of 2025 on file reported 5,391 arrests to the FBI that year. Those kept in custody were booked into the county jail; a person cited and released at the scene was not.
The counts, agency by agency and year by year, are in Mountainside arrest records.
From the Jail to State Prison
A sentence of more than a year is served in state prison, not the jail; the New Jersey Department of Corrections lists its inmates at www20.state.nj.us/DOC_Inmate/inmatefinder?i=I.
Frequently Asked Questions
How do I find an inmate in Mountainside?
Look on the custody page of the Union County Sheriff's Office at unioncountynj.org. It covers people booked into the county jail.
How many people are in the Union County jail?
About 330 on an average day in 2020, the latest year in the Vera Institute's jail data, against a rated capacity of 995 beds.
Where do people sentenced to prison in Union County go?
To a facility of the New Jersey Department of Corrections; its inmate locator is at www20.state.nj.us/DOC_Inmate/inmatefinder?i=I.
Is this the official Mountainside website?
No. SearchSystems.net is an independent public records guide, online since 1997. It links to the official city, county, court and state offices described above and does not hold records, charge for them or take requests.