Diamond Lake is a glacial-origin lake located in the village of Mundelein in northeastern Lake County, Illinios. The lake has an average depth of around 7.61 feet (2.32 m), a maximum depth of 23 feet (7.0 m) a shoreline of 5.9 miles (9.5 km), and a surface area of 153 acres (62 ha). The lake's watershed contains about 686.16 acres (277.68 ha) of residential and agricultural land surrounding the lake. Diamond Lake's water is replenished by precipitation and runoff within its watershed. Water exits the lake via a drainage pipe that eventually connects to the Des Plaines River.

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The Mundelein Park District gained ownership of the majority of the lake in the 1960s. William Fenwick builds his first house on the south bank of Diamond Lake, near present day Oak Terrace in 1835. In 1836, A massive 50-year settler migration begins after Native Americans are required to leave Illinois. In 1840 Alexander Bilinski purchases north side land and builds his first store and taverns.

Carved by retreating glaciers about 14,000 years ago. That history is now on display through “Lost and Found Artifacts of Diamond Lake,” a special exhibit at the Mundelein Heritage Museum.

Diamond Lake is home to several species of flora and fauna. The most common aquatic plants found in the lake include coontail, sago pondweed, and the invasive Eurasian watermilfoil and curly-leaf pondweed. In 2008, the following thirteen species of fish were identified in the lake: bluegill, pumpkinseed, largemouth bass, yellow bass, yellow perch, black crappie, walleye, northern pike, channel catfish, Eurasian carp, golden shiner, emerald shiner, and American gizzard shad.

In 2008, invasive zebra mussels were first sighted in Diamond Lake.

Common loons and American white pelicans can be sighted at the lake during their annual migrations. Canada geese, mallards, and great blue herons are common residents, and bald eagles are often seen flying over the lake. Muskrats, red foxes, and painted turtles also live in or around Diamond Lake.

Residents living near the lake founded the Diamond Lake Preservation Association in 2014. The group's mission is the maintain the health and usability of Diamond Lake by effectively managing its invasive species.