One of the biggest lessons Kwame Jackson learned in his three years as a stockbroker at Goldman Sachs Group Inc. helped get him fired by "The Donald."

Mr. Jackson, 30 years old, came in second out of more than 200,000 contestants vying to be hired by Donald Trump on the hit reality-TV show "The Apprentice." In the show's final episode, Mr. Trump said he passed over Mr. Jackson largely because of his laid-back style. Mr. Trump selected a more rah-rah manager to be his apprentice.

Mr. Jackson, who left Goldman to appear on the show, says the elite Wall Street investment bank taught him: "You can do things quietly, humbly, and still be successful." He adds: "I didn't have anything to prove to anybody. ... In the end it was up to Trump."

Goldman prides itself on its low profile; its downtown Manhattan headquarters here doesn't have a sign noting the firm's presence. Mr. Trump, on the other hand ... well, his self-promotion is legendary: His name is on everything from helicopters to casinos to bottles of water.

"The two firms are about as polar opposite as two things could be," says Roy Smith, a professor of finance at New York University and a former partner at Goldman. Although Mr. Smith was on sabbatical in Barcelona for the past several weeks, he said he had heard of the show. "That is the show where Trump fires people, right?" he said. "Did a guy from Goldman really give up his job for that?"

He had little choice: Last summer the firm turned down Mr. Jackson's request for a leave of absence to appear on the show. "I was told it was too much of a reputational risk for me to go on the show while a Goldman employee," Mr. Jackson says. "Senior management basically thought I was crazy."

Goldman has long shunned those who seek the spotlight. In the mid-1980s, James Cramer, a former Goldman trader turned hedge-fund manager and TV commentator, found himself in hot water after his photo ran in the New York Times with a caption suggesting he could buy anything he wants. "Life was never the same after that article," Mr. Cramer writes in his book, "Confessions of a Street Addict." He confessed: "I had broken the code." Mr. Cramer left Goldman a few months after the article appeared.

Yesterday Mr. Trump said that while Goldman's approach to business is quite different than his, he actually likes the firm's low-key approach to things: "It is not a bad idea, but it has never worked that way in my firm. I should try it."

Several Goldman staffers have requested leaves to appear on reality-TV programs in recent years. So far all have been given the cold shoulder. "We can understand why people want to do this, but being thrust into the limelight is the antithesis of the way we do business," says a Goldman spokesman. "That said, Kwame had a lot of fans rooting for him here."

Mr. Jackson's decision took some people by surprise for other reasons. A number of traders at the firm make in excess of $10 million a year. As a securities broker, Mr. Jackson earned an estimated $125,000 a year, according to a person familiar with the matter. Mr. Jackson's decision to leave Goldman took even Mr. Trump by surprise, who said on one of the show's episodes that Mr. Jackson gave up so much "it scared him." The apprentice's job pays $250,000 a year.

Now, after having a taste of Mr. Trump's world, Mr. Jackson has two words for the ways of Goldman: You're fired! "If I wind up back at Goldman then the plan has gone awry," he says. "I'm focusing on the commercial opportunities related to the celebrity world as well as starting my own company."

He says one of those opportunities is a job offer from Mark Cuban, owner of the Dallas Mavericks, to run one of his investment portfolios. He says the offer was made after the show's wrap-party Thursday night. "I don't know how many times you can get a billionaire to wait for you at your party," he says. "Maybe Warren Buffett will be at my next party."

Mr. Jackson says he did get valuable experience at Goldman. Managing money at Goldman for wealthy individuals prepared him for the challenges that Mr. Trump threw his way. "You learn to present yourself in a fashion that really helps you to seem reputable as well as trustworthy as well as confident," Mr. Jackson says. "So when you're sitting in the boardroom with Donald Trump, it's not the first time you have been in front of someone who is wealthy and powerful. You're not intimidated."

His style hurt him most when it came to dealing with cast member Omarosa Manigault-Stallworth, whom Mr. Jackson picked to be on his team in one of the final episodes despite her track record on the show as a schemer. She lied to him, costing him points with Mr. Trump, who said Mr. Jackson should have "fired her or severely reprimanded her" after this incident. Mr. Trump ended up picking micromanager Bill Rancic, who previously ran an online cigar company, to be his apprentice.

"I have always worked around very skilled and competent people," Mr. Jackson says. "You had to work with what you had and that is what I did."