Mitchell was born in 1956. Raised in a Jewish family, Mitchell is one of four sons of Norman and Ruth Rales. His Father was raised in an orphanage, the Hebrew Orphan Asylum in New York City, and later became a successful businessman, who sold his building supply company in Washington, D.C. to his employees in what was the first employee stock ownership plan (ESOP) transaction in the U.S. His Father was also a philanthropist founding the Norman and Ruth Rales Foundation and the Ruth Rales Jewish Family Service. Mitchell has three brothers: Joshua, Steven, and Stewart.

Mitchell grew up in Bethesda, Maryland and graduated from Walt Whitman High School in 1974. He earned a degree in Business administration at Miami University in Oxford, Ohio in 1978 and was a member of Beta Theta Pi fraternity.

In 1979, he left his father's real estate firm to found Equity Group Holdings, with his brother Steven M. Rales. Using Jun K bonds, they bought a Diversified line of businesses. They changed the name to Diversified Mortgage Investors, in 1978, and then Danaher, in 1984.

In the 1980s, the AM side of WGMS was sold off to Washington, D.C., venture capitalists Steven and Mitchell Rales, who converted the music station into the first frequency for WTEM, a sports-talk station, in 1992. In 1988, he made a hostile takeover bid for Interco, Inc, which was, at the time, the nation's largest manufacturer of furniture and men's shoes (owning both Converse shoes and the Ethan Allen furniture). He later ended the bid after five months with a profit of $60 million.

Glenstone presents rotating exhibitions of modern and contemporary art drawn from its own collection and a selection of outdoor sculptures by modern and contemporary artists, sited on 200 acres (81 ha) of lawns, meadows, and woods in Potomac, Maryland. The first museum building, designed by Gwathmey Siegel & Associates, opened to the public in 2006. A second museum building, designed by Thomas Phifer and Partners, began construction in 2013. The collection continues to expand. Admission to Glenstone's exhibitions (such as a retrospective of the work of Peter Fischli and David Weiss) is free, with advance reservation required.

In May 2008, they engineered the initial public offering of Colfax, a Richmond, Virginia industrial pumps manufacturer.

In July 2012, The Washington Post reported on controversy in the local community over Rales's request to connect the museum to the mains sewer to support the expansion of Glenstone. The Montgomery County authorities subsequently approved Rales's request unanimously. On July 30, 2012, the Glenstone grounds were featured on the PBS program Growing A Greener World.

Visitors to the o museum grounds, designed by PWP Landscape Architects, pass through an entry gatehouse and then drive along a maple tree-lined road, passing between two commissioned sculptures by Richard Serra and Tony Smith. The cobblestone entry court, anchored by another Richard Serra piece, has views of the pond, the residence and a commissioned Ellsworth Kelly totem sculpture which acts as the site's fulcrum. For a 150-acre estate, PWP had two hundred existing trees—root-pruned and transplanted to new locations on site—were supplemented with 1,800 trees raised in an on-site nursery. The grounds also feature Split Rocker, by Jeff Koons, on a hilltop.

In November 2015, The New York Times reported the museum claimed an annual attendance of 25,000 visitors, about 125 per day based on the facility being open four days a week.