Waukegan's proximity to Lake Michigan helps cool the city throughout the year.

Waukegan started as a French trading post known as "Little Fort". Proud of the growth of their community and no longer wanting to be characterized as "little," residents changed the name of their town on March 31, 1849, to Waukegan, the Potawatomi word for "fort" or "fortress".

Ray Bradbury, best known for his novel Fahrenheit 451 (1953) and his short-story collections The Martian Chronicles (1950) and The Illustrated Man (1951), was born in Waukegan. Although he moved with his family to the West Coast while still a child, many of his stories such as Dandelion Wine (1957) explicitly build on Waukegan (often called Green Town in his stories) and his formative years there. Ray Bradbury Park, located at 99 N. Park Avenue, is named in his honor.

During the 1920s, organized crime boss Johnny Torrio helped build the Chicago Outfit, which was later inherited by his protégé Al Capone. When Torrio served time in Waukegan's Lake County jail in 1925, he installed bulletproof covers on the windows of his cell at his own expense for fear of assassination.

Waukegan has a total area of 24.50 square miles (63.45 km²), of which 24.26 square miles (62.83 km²) is land and 0.24 square miles (0.62 km²) or 0.99% is water.

Waukegan is the hometown of comedian Jack Benny (1894-1974), one of the 20th century's most notable and enduring entertainers, but although he claimed for decades on his radio and television shows to have been born there, he was actually born at Mercy Hospital in Chicago. His family's home at 518 Clayton Street was designated a landmark by the city on April 17, 2006.

On Christmas morning in 1984, a fire broke out in the Karcher Hotel, a historic building at 405 Washington Street. Eight people died from carbon monoxide poisoning, seven were injured, and four were hospitalized. Today, guests often report hearing disembodied screaming voices and the unexplained smell of charred smoke. Others have reported seeing shadowy figures, as well as objects moving on their own, faucets turning, and doors slamming shut. The fifth, six and seventh floors are said to be especially haunted, with guests describing the sensation of invisible people sitting next to them on beds and being touched by unseen hands.

Of the 54.4% of the population that identify as religious, the most populous group is Catholic at 31.0%. Other common affiliations include 3.2% Lutheran, 2.7% Jewish, 1.9% Baptist, 1.6% Presbyterian, 1.5% Methodist, and 1.4% Islamic

The record high temperature of 108 °F (42 °C) was set in July 1934, and the record low of -27 °F (-33 °C) was set in January 1985.

Ann Taylor became the city's first female mayor in April 2021 when she defeated incumbent Sam Cunningham, the city's first African American mayor.