“I’m humbled by where I’ve come to,” said Melissa Cohn, founder and CEO of the Manhattan Mortgage Company, the biggest residential mortgage brokerage in the tristate region. “But I also understand that success is fleeting.”

Both statements lend themselves to analysis. Ms. Cohn doesn’t project humility. She’s not a retiring type. She’s enormously self-confident. She responds to questions with candor and directness. She’s quick on the uptake. There are few shadings or qualifications to her assertions.

And her success has scarcely been fleeting. She’s sustained her business for 20 years. Almost single-handedly, Ms. Cohn grew her company’s sales from $30 million in its first year, 1985, to over $4 billion in 2004. Ms. Cohn arguably has enjoyed success since age 9, when she sold brownies during summer vacations on Martha’s Vineyard.

“I’m a born entrepreneur,” she said. “I hate to lose. I take intense pride in what I’ve created. I love numbers, and I’m very good at problem solving. I was always determined to succeed, even at an early age.”

Although she demonstrated entrepreneurship on Martha’s Vineyard, opening a riding school at 12, Ms. Cohn only discovered her love of numbers at Smith College. She initially wanted to be a doctor and took pre-med courses; she’d wanted to follow in the footsteps of her great-grandfather, the acclaimed Berlin-born biologist and physician Leonor Michaelis, who made a name in the study of enzyme kinetics. But she was stymied by organic chemistry.

She majored in American studies. After graduating in 1982, she went to work at a Citibank branch at Lexington and 86th. The manager suggested she focus on mortgage lending because it was on the rise.

“I learned the business from the ground up,” Ms. Cohn said. “It was also a great way to learn about New York City. And it was a great way to learn about people. After all, a mortgage is generally the single largest financial transaction in a person’s life.”

But how did she acquire her knowledge of the arcane aspects of mortgage lending?

“I winged it,” Ms. Cohn said. “I was constantly using my wits.”

She stayed at Citibank for 18 months before moving to a large brokerage. There she expanded her network of contacts in the banking and real estate communities. She brought in business for the company. But one day, about 18 months into her job, the owners told Ms. Cohn that her $30,000 salary was too high and would be trimmed.

“I was incensed, to say the least,” Ms. Cohn said. “I decided there and then that I would create my own mortgage business.”

Her first mortgage loan was drafted on the dining table in her apartment. A friend at an investment bank wrangled some space at his Park Avenue office for her to hang her shingle. The name she chose for her business, the Manhattan Mortgage Company, was intended to suggest that “the business had been there forever,” she said.

Her business model was predicated on being a go-between for banks and individual customers, particularly those seeking luxury residences. Ms. Cohn cannily recognized that, besides first time mortgages, there would be a boom in home-loan refinancing as interest rates dropped to record lows. She also recognized that large institutions that loaned money, such as banks, were looking to cut processing and staff costs.

“That’s where Manhattan Mortgage came in – we made it easier for the banks as well as the customer,” Ms. Cohn said. “And instead of waiting for business to develop, I went out and created the curve. I anticipate what the market will do.”

That feel for the market has served her well. The Wall Street Journal recently quoted the managing director of Wholesale Access Mortgage Research & Consulting, David Olson, as saying that of the $2.5 trillion in mortgages taken out last year, roughly 60% was handled by the nation’s 120,000 brokers, up from 20% in 1987.

Ms. Cohn acknowledged that she was the beneficiary of major changes in the mortgage business. In the 1970s and ’80s, she said, the sector was dominated by savings and loans offering mostly 15-30 year mortgages. From the late 1980s through the ’90s, it grew into a high-volume national market, she said. Her company expanded into New Jersey, Connecticut, Vermont, Massachusetts, Colorado, Florida, and California.

The transformation of mortgage lending into a national phenomenon meant that big lenders such as Washington Mutual, Wells Fargo, and Countrywide Financial expanded their use of brokers like Ms. Cohn. Indeed, most of her big lenders are not based in New York.

“These institutions were looking for a low-cost way to get into broader markets,” she said. Manhattan Mortgage undertakes all the appraisals, credit checks, and loan processing.

Typically, a broker gets the equivalent of about 1% of a mortgage. “It is paid by the bank so that the rates and products we offer are never higher than what a buyer can get from a bank directly,” Ms. Cohn said. “Oftentimes, we can offer a better rate by working on a tighter margin.” The range of Ms. Cohn’s mortgages is from $75,000 to $10 million, with most loans taken to buy or finance luxury residences.