Rosaryville Park was established in 1973 under the authority of the State of Maryland's Department of Natural Resources, but it was not until improvements were added in 1989 that public use began. In 1990, the Rosaryville Conservancy—a NFP corporation—was formed by local citizens who had an interest in seeing the Park used to promote and preserve Southern Maryland's rural, colonial, agricultural, and equestrian heritage.  A year later, in 1991, the Conservancy established the Marlborough Horse Trials as an annual event for raising funds to help preserve and protect the Park.  Subsequently The Marlborough Horse Trials, Inc. (MHT)—another not-for-profit corporation—was formed to continue support the equestrian sport of eventing at Rosaryville.

Unlike many other local state parks that are long and narrow stream valley parks, Rosaryville is a roughly rectangular upland area without significant water features.  The land typifies Atlantic Coastal Plain and the rolling fields and forest of Southern Maryland.  Wide-open fields encourage wildlife.  Woodland and open land species are found. Retaining mixed habitat helps conserve both types of wildlife. The topography includes some steep slopes, small knolls, and deep ravines, with elevations ranging from 68 to 256 feet. It is an ideal setting for horse trials

The horse trials were conceived by a core group of founding families, farms, and businesses in the area. The Marlborough Hunt Club, raised the funds and provided supplies to get the event up and running and initial construction funds for the cross country course were also provided by a grant from the Chevrolet Motor Division of General Motors.

The MHT cross country course ranks with the best. The course is designed by Roger Haller (Oxford, GA) and constructed by Chris Milanesi (Old Chatham, NY).  This pair is renowned for their expert work at other eventing venues, including the 1996 Atlanta Olympics.   Since its inception, the horse trials have become a major equestrian and community event in the Southern Maryland region.

Rosaryville State Park is also home to historic Mt. Airy Mansion. Charles Calvert, Third Lord of Baltimore, constructed the original wing of Mt. Airy in the late 17th century as his hunting lodge.  Mt. Airy was the site for the 1774 wedding of Eleanor Calvert and John Parke Custis, the stepson of George Washington who attended the wedding and was a frequent houseguest.  The State of Maryland bought the property in 1970, assisted with Federal matching funds.  Mt. Airy exemplifies the early American manor house and provides history of 250 years of farming and rural living in Southern Maryland.

In 1991, the Conservancy entered into a use agreement with the State of Maryland to oversee and operate a portion of the park, including Mt. Airy Mansion and designated acreage.  The State has provided a gated entrance and paved access road from US 301 South, along with some storm water drainage and erosion control improvements.  Additionally, three picnic areas were established as part of the paved road network, and two pavilions with restroom facility were only built. 

Open space is maintained thanks to the efforts of MHT and others.  Potomac Electric Power Company (PEPCO) has a high-voltage transmission tower right-of-way through the park, and the Washington Suburban Sanitary Commission (WSSC) has a water main right-of-way that runs under the park. In addition, bicyclists from Maryland Off-road Enthusiasts (MORE) maintain a circumferential trail.

The park is open to the public for weekend use May through early September except when reserved for horse trials events and work days.  There is no on-site park ranger, although a DNR employee occupies a tenant house at the park entrance.

The Marlborough Horse Trials are sanctioned by United States Eventing Association (USEA) and USA Equestrian (USAE). Together, these organizations establish standards for competition in the US and oversee development of the sport to prepare riders to represent the U.S. at the Olympics and other world class events.  MHT has hosted the Mid-Atlantic (Area II) Novice Division Championships three times. Successful competitors earn points toward national level awards.