ROBERT P. KOGOD, a prominent area businessman, has purchased the Vanderbilt mansion at 2929 Massachusetts Ave. NW, paying $2.12 million to a New York investment group.

The 40-room mansion, compete with ballroom, sold at an auction for $3.1 million last year to an unidentified business, which backed out of the deal weeks later and forfeited a $310,000 down payment. The mansion's owner of two years, New York Land Co., put the property up for sale again shortly afterward with an asking price of $2.3 million.

"A property like this is so unique, it is almost impossible to tell what it is worth," real estate agent Marvin Hill said about the $1 million difference in sale price. "Obviously, we are all very pleased with this second transaction."

The mansion, built at the turn of the century and owned for decades by Gladys Vanderbilt, most recently was used by the Saudi Arabian government to house embassy personnel. It is in a residential zone with a diplomatic overlay that would allow it to be used as an embassy or attache' building. The old mansion looks unkempt after two years of disuse, but Hill said it is structurally sound.

When contacted this week, Kogod would say only that he and his wife plan to move to the Embassy Row house from their current Bethesda home sometime in the near future.

"I believe he wishes to keep as low a profile as possible," said Hill, who handled the transaction.

Kogod is president of Charles E. Smith Management Inc., a real estate company based in Crystal City, and is on the board of trustees of American University, where the Robert P. Kogod School of Business was named after him in 1979. A university spokesman said Kogod is an alumnus of the school and a benefactor.

THE $500 MILLION sale of 45 prime buildings with nearly 6,200 apartments in Manhattan, Queens and Long Island may be the biggest residential real estate deal in New York City history, its participants said.

Ted Hearne, spokesman for the Chicago-based John D. and Catherine T. MacArthur Foundation, best known for its "genius grants" to promising individuals in various fields, said the sale of the New York property was part of a move by the foundation to "reorganize their properties."

The foundation released a statement last week saying it had contracted to sell 10 properties jointly to Mendik Co. and Martin J. Raynes for conversion into cooperatives and condominiums. The statement called the deal "one of the largest residential real estate acquisitions in New York City history."

The prime property in the package is Lincoln Towers, eight buildings containing 3,859 apartments near Lincoln Center on the west side of Manhattan. Seven other Manhattan properties are along the East Side from 54th to 78th streets. The Queens building is at 125-10 Queens Blvd., and the Long Island property is at Great Neck Terrace in Great Neck.

The properties include 6 million square feet of residential space and 120,000 square feet for retail operations, and they cover more than 60 acres of land.

The portfolio sale is one of three major sales, totaling $1.3 billion, made by the foundation in the last 13 months, said its president, John E. Corbally. The others included the sale of 19 Manhattan office buildings for $400 million and Banker's Life and Casualty Insurance Co. for $380 million.

Bernard H. Mendick, the third-largest owner of commerical property in the city, is the managing partner for the new owners, and Raynes, a specialist in apartment conversions, will be sales and managing agent, the statement said.

IN THE BUSINESS . . . 1st Washington Mortgage Corp. has formed a joint venture with Atlantic Federal Savings of Baltimore to offer home and commercial mortgages in Baltimore and Annapolis. 1st Washington is itself a joint venture of Washington Federal and Columbia First Federal Savings . . . The Columbia Technical Institute in Arlington is offering courses in construction estimating, electrical, heating and air conditioning systems, and other building-related subjects. For information, call 920-8866 . . . Dreyfuss Realtors is opening a general brokerage division to concentrate on residential resales. Earle D. Hightower has been named sales manager . . . The 233-room Park Hyatt is under construction at 24th and M streets NW. The hotel is the second phase of a $200 million joint venture of Boston Properties and U.S. News & World Report; Omni Construction is the general contractor . . . Omni's sister firm, George Hyman Construction Co., has begun work on One Westin Center, a 400,000-square-foot office building at 2445 M St. NW. It will be joined by a courtyard and canopy to the 416-room Westin Hotel, for which Hyman also is the contractor. Oliver T. Carr Co. is the developer.

PERSONNEL FILE . . . Francis C. (Fran) Steinbauer, now president of the eastern division of Mobil Land Development Corp., has been named president of Westfields Development Corp., a subsidiary of Henry A. Long Co. Westfields is a 1,000-acre mixed-use development planned along Route 28 near I-66 and Route 50 in Western Fairfax . . . Gerald D. Hines Interests has promoted William D. Alsup of its Washington office to vice president . . . Ralph Tasman has been named vice president, special projects, of Rozansky & Kay Construction Co. . . . C. Cammack Morton has been appointed president and chief operating officer of the retail development division of Western Development Corp. . . . Edward J. Miller Jr. has been named executive vice president of W. C. and A. N. Miller Development Co., and Robert R. Miller and Allison N. Miller III have been named vice presidents . . . The Mortgage Bankers Association of America has announced three promotions: Warren J. Dunn to senior staff vice president, communications; Glen S. Corso to senior staff vice president, residential finance and government agency relations, and Michael D. Davis to senior staff vice president, administration . . . Michael Was, executive vice president of Reston Land Corp., was cited as "Marketing Man of the Year" by the National Association of Home Builders at its recent convention . . . Jenny Cooper has joined Mount Vernon Realty's Relocation Center as director of relocation, and JoAnn Stewart has joined the firm as marketing manager . . . Maren Auerbach has been promoted to vice president of Realty World Corp. . . . Stepheny A. Madsen has been named vice president and executive director for ALDA Councils at the American Land Development Association.