A unique mixture of coastal and tropical hammock and mangrove forest, this barrier island provides a haven for several rare or endangered native tropical and coastal plant species. The park's Nature Center shows visitors why the park is a biological treasure. Visitors can swim, picnic, and surf at the beach; scuba diving and snorkeling are also popular activities. Birdwatchers can see herons, brown pelicans, terns, sandpipers, and gulls. Anglers can fish in the lagoon by wading, kayaking, or canoeing. They can also fish from non-swimming areas of the beach. Located in northern Palm Beach County, 2.8 miles south of the intersection of U.S. 1 and PGA Boulevard on A1A.

In addition to numerous species of shore and wading birds, the park is a prime nesting site for sea turtles. Large numbers of loggerhead, green and leatherback turtles nest from early May through late August.

The earliest evidence of human occupation of this area comes from artifacts recovered from "kitchen middens" located in the park. Native Americans who settled this area, gathered food from the ocean and lake. Fragments of bones, shell and pottery were discarded in refuse piles or "middens."

In the early 1900s, Munyon Island was famous for its lavish resort hotel, "The Hygeia." Built by Dr. James Munyon, the hotel was destroyed by fire in 1915. Munyon Island is not open to the public, but only accessible by boat.

The park is named after the previous property owner, John D. MacArthur, who wished to preserve it for future generations to enjoy. In the 1970s, after a university study convinced MacArthur that the property was a biological treasure, he donated a section for use as a public park. The John D. and Catherine T. MacArthur Foundation contributed additional funds to help develop the park and Nature Center. Facilities opened to the public in 1989.

The William T. Kirby Nature Center features displays depicting the park's natural communities and live animal exhibits. Visitors may also view a 15-minute video on the park. The Nature Center is open daily from 9:00 a.m. until 5:00 p.m.

Scuba diving and snorkeling are permitted. A "Diver Down" flag, for snorkelers and divers, is required by law and is available for rent at the Nature Center.