Since October 25, 1949, several land acquisitions have increased the size of the monument. Most recently, in December 2000, the Heritage Addition added 1,045 acres to the monument, increasing it to 2,526 acres. There are more than 200 known prehistoric mounds, constructed between 2,500 and 700 years ago, within Effigy Mounds National Monument.
The monument consists of the North, South, Heritage Addition, and Sny Magill Units, with the largest concentration of mounds at Sny Magill. In addition to the mounds, there are 18 rock shelters in the North and South units of the monument. Historic period resources include a remnant section of a (National Register of Historic Places eligible) military road constructed in 1840 and archeological remains associated with various historic period activities.
The monument curates an extensive collection of more than 8,000 prehistoric artifacts and more than 12,000 manuscripts that are displayed in an interpretive museum and stored in the archives. The archives also contain a number of biological, geological, and paleontological samples.
Park administrative offices are housed in the houses on the west end of the parking lot. Upon passage of the National Historic Preservation Act of 1966, the monument, along with many other National Park Service sites and properties, was listed in the National Register of Historic Places.