Douglas Jemal and his family have been working since the summer of 2000 to buy up parcels at Mount Vernon Square, east of the Walter E. Washington Convention Center, in hopes of turning it into a key part of their D.C. real estate empire.

Their first deal involved a string of properties along New York Avenue NW. The purchase price was $3.1 million, according to property records.

Over the next decade, Jemal, his son Norman and staff bought nearly the entire triangular-shaped block bordered by New York Ave. to the north, K Street to the south and 7th and 6th streets to the west and east, and much of the block to the north as well.

Last week the work paid off when Douglas Development sold much of the southern block to the Association of American Medical Colleges for $57 million. The association plans to erect a nearly 300,000-square-foot new office building, where it will relocate its headquarters from the West End.

Jemal’s Mount Vernon Square holdings stand out for the perseverence that was required to complete about a dozen land deals. The final holdout on the southern block, the owner of a townhouse at 628 New York Ave., finally agreed to sell to AAMC this year. (Norman Jemal on the purchase price: “He did very well. He should be living in a much nicer home when he’s done with this.”)

Merrick Malone, president of the D.C. Building Industry Association, says Jemal’s success was built on his patience. “One of the things Doug has always demonstrated a skill for is patience … and that’s the thing about assemblages, they do take time. And you have to have the will to understand that, and you have to have the financial wherewithal and the patience to do that.”

Norman Jemal said it’s hard to calculate how much the company earned in the deal, which includes vacant lots, an auto body shop and a gym. But the deal sticks out for a number of reasons. First, the Jemals plan to buy back the retail space in the bottom story of the new association building. Second, they will physically relocate four historic buildings from the footprint of the office building in order to preserve them.

Lastly, the Jemals are notorious for buying and hanging onto properties — consider the Uline Arena in Northeast D.C. and former Wonderbread Factory in Northwest — and rarely selling. But Norman said the portfolio had become too much to handle.

“We have more development opportunities than we have the capacity to do. My dad’s been buying property in Washington D.C., saying it’s the capital of the country and the capital of the free world for years and years, since nobody wanted to be here.”