Considered to be part of the Ridge and Valley Appalachians, the mountains, which reach elevations over 1,100 feet, form an imposing geologic barrier just west of the Hudson River.

The Marlboro Mountains form a broken ridgeline running from the Ulster–Orange County border in the south to the mouth of Rondout Creek in the north.

North of this section is Shaupeneak Mountain, which bends to the north-northwest before reaching an area known as The Hell, a gap in the ridge through which a creek flows.

North of this gap is Hussey Hill, which stretches to an area west of Port Ewen, New York, marking the northernmost limit of the Marlboro Mountains.

[6] Though occupied and utilized by people for thousands of years, the Marlboro Mountains, which have historically provided timber, game, and agricultural land, were more intensely exploited for their resources in the past two centuries than at any other time in human history.

It is in the Marlboro Mountains that naturalist and essayist John Burroughs, with the aid of his son Julian, constructed Slabsides, a cabin retreat which he used in the summer.

A series of infamous events in the history of the Marlboro Mountains and the town of Esopus involves gold mining operations at Hussey Hill.

The Dimmicks would lead Eltinge and others on fruitless gold finding expeditions in the Shawangunks and Catskills, leaving their followers broke.

[8] In 1879, the owners of the land on Hussey Hill granted a lease to Simon S. Westbrook, who had purchased the Hudson River Gold & Silver Mining Company's equipment at foreclosure.

In spite of this fact, Burger managed to sell a twenty-year lease to three men hoping to strike gold on his property.

[10] The Marlboro Mountains are primarily formed of shallow marine/delta sediments deposited during the late Ordovician, approximately 450 million years ago.

The Marlboro Mountains provide numerous outdoor activities to the Mid Hudson Valley, including hiking, cross country skiing, and fishing.

[12][13] Recreation areas, nature sanctuaries, and trails from north to south: Recently, new land has been preserved on Illinois Mountain by the Scenic Hudson organization.