In 1913, Hillside voted to secede from Union Township. In the years that followed, the population and industry of the town grew from a rural community to an industrial suburb. In the 1920s, the township would prove to be a solution to the post-war housing crisis as its location between two cities would cause the population to grow from 5,400 to 17,601 during the decade.

In the following decades, Hillside would become the home of Bristol-Myers Squibb and Lionel Trains which served as major employers for the community with the town reaching an economic peak in the 1960s. 

The Westminster neighborhood served as the former home of the Pingry School which later became the East Campus of Kean University. It is in this area that the county would establish Conant Park, which at the time was bounded by the Elizabeth River, Conant Street, and the Kean Estate, the former home of Governor Thomas Kean. 

Today, Hillside is a densely populated community with central industrial centers located along Route 22, Interstate 78, and the railroad. The Township of Hillside has a total area of 2.78 square miles and is bordered by fellow Union County towns of Elizabeth and Union to the east and west and being the northernmost township in the county, is bordered by Newark and Irvington of Essex County to the North. With its close proximity to Elizabeth, Newark, and Union Township, Hillside for many years had no true business center with Liberty Avenue serving as a hub for the town’s retail and food services.

Hillside has grown slightly since its population boom in the 1920s with over 22,000 documented residents as of the 2020 Census which ranks it as the 124th most populated city in New Jersey. According to this most recent Census data, of those 22,000 residents, 51.5% identify as Black, 20.8% as Hispanic, and 17.7% as White. The median household income for Hillside was reported as being $78,779.