A Montana cattle ranch co-owned by billionaire Whitney MacMillan has been listed for $18 million.

Mr. MacMillan is the former CEO and a member of the founding family of the agribusiness giant Cargill Inc. He owns the roughly 40,000-acre PN Ranch near Winifred, Mont., with partner Matt Pierson of Highland Livestock. The property is being listed by Tim Murphy and Dave Johnson of Hall and Hall.

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The Montana ranch has been listed for $18 million.
Kenton Rowe

The ranch is being sold with all its cattle and equipment included, said Mr. Pierson, whose family purchased the ranch in the 1990s, with Mr. MacMillan becoming a partner several years later. Currently there are about 800 mother cows on the property as well as wildlife and bighorn sheep.

At the confluence of the Missouri and Judith rivers, the ranch is one of the oldest in Montana and has a notable history, Mr. Murphy said. The site was once a busy hub of steamboat traffic along the Missouri River, and in the late 1800s the first military outpost in the Montana Territory was set up there.

The ranch represents “a large piece of Montana history,” said Mr. Pierson.

Buildings erected around that time are still standing on the site, including a granite and sandstone warehouse, a post office, a school house and a “mansion house,” Mr. Murphy said.

The roughly 3,136-square-foot, two-story house, built in the 1880s, stands in a grove of cottonwood trees by the river and isn’t currently inhabited, he said. But the owners added a new roof and siding to stabilize the structure, Mr. Pierson said, noting that it could now easily be made habitable if the new owners wanted to live there.

There are three other homes on the property used by employees and by the owner and guests.

Mr. Pierson said he is selling because his father and brother passed way recently, and he has his hands full running his family’s other ranch in Livingston, Mont. Mr. MacMillan also has other ranches, he said.

Write to Candace Taylor at Candace.Taylor@wsj.com