Hillside was incorporated as a township on April 3, 1913, from portions of Union Township, based on the results of a referendum held on April 29, 1913.
Hillside was created from parcels of land carved out of neighboring Newark, Elizabeth, and Union. It originally contained the farms of Woodruff, Conant and Saybrook. Local streets still bear their names.
Hillside was incorporated shortly after the appearance of Halley's Comet in 1910, and for that reason, the team nickname of Hillside High School is the "Comets." Several local businesses take the name "Comet" for the same reason.
The Hillside Historical Society was established in the 1980s in the Woodruff home on Conant Street, perhaps the township's oldest. The Woodruff House and Eaton Store Museum is operated and maintained by the Hillside Historical Society. Purchased by the Society in 1978, the house has been faithfully restored to its original grandeur. The Woodruff House spans three centuries in one structure, from the original 1735 building, to the 1790 addition, to the 1890s kitchen all the way to the 1900s store. The society has also added to the grounds an authentic post and beam barn, a Phil Rizutto and All Sports Museum honoring the Hillside legend as well as an archive to house the many documents the society has obtained over the years.
Jean-Ray Turner, a reporter for the Elizabeth Daily Journal, wrote Along the Upper Road, in the 1970s, a book of the history of Hillside.
Hillside has been the home of Bristol-Myers Squibb and for years was the site of the Lionel Trains factory. The town thrived for decades and reached an economic peak in the 1960s. Blue collar workers who lived primarily in the central part of town were employed in local manufacturing concerns. White collar workers established the neighborhood known as Westminster where Yankee shortstop and broadcaster Phil Rizzuto lived for most of his adult life, until his death. That section of town also included the private Pingry School for boys, which became co-ed in 1974.
In the 1950s and 1960s the township was approximately one-half Jewish, many of whom lived either in Westminster or in the area of Hillside near Chancellor Avenue, adjacent to the Weequahic, section of Newark. This section of Newark was the early home of comedian Jerry Lewis and writer Philip Roth ("Portnoy's Complaint").
In the early 1950s the township established Conant Park, its largest. The park is bounded by the Elizabeth River and Conant Street. At the rear area of the park near Pingry School was the boundary of the Kean Estate, the boyhood home of Governor Thomas Kean (1982–1990). The wealthy Kean family also donated the land on Morris Avenue and helped to establish Newark Normal College in 1885, which was renamed Kean College, and later Kean University, in the family's honor. Also in the 1950s the Town Hall, Police Headquarters and Municipal Library were constructed at the corner of Liberty and Hillside Avenues.
Popular township organizations include Rotary International, Kiwanis, Knights of Columbus, Elks, the Hillside Industrial Association, the Hillside Business and Professional Women's Club, the Republican Club and the Democratic Club, as well as a number of ethnic clubs and associations.
In 1991, police from both Hillside and Newark fired nearly 40 shots at a van that had rammed a Hillside police vehicle after a high-speed chase. The pursuit had started after the van had been reported stolen at gunpoint in Newark and was being followed by three Newark police cars before crossing into Hillside. Two of the people inside the vehicle were killed and four of the five other passengers were wounded, though the Union County Prosecutor indicated that there was no clear explanation for why the police had started shooting. The Reverend Al Sharpton held a rally outside Town Hall on Hillside Avenue demanding that the police officers involved in the shootings should be prosecuted for their actions.