Prior to joining MarketWatch.com, Greenberg was senior columnist for TheStreet.com for six years.

Before transitioning to online journalism, Greenberg spent 10 years as the six-day-a-week columnist for the San Francisco Chronicle's Business section. While at the Chronicle, for several years in the mid-1990s, he also had his own America Online business commentary site, Bizinsider. Also while at the Chronicle, Greenberg spent several years as the morning business reporter for KRON-TV in San Francisco and freelanced for five years as the monthly "Against the Grain" columnist for Fortune .

Earlier, Greenberg was a New York-based financial correspondent for the Chicago Tribune after transferring from its Chicago newsroom, where he covered the food and restaurant industry. Greenberg has also worked for Crain's Chicago Business, the St. Paul Pioneer Press, Amusement Business, and the Boca Raton News.

Greenberg is an alumnus of the University of Miami with a bachelor's degree in journalism. He now resides in Connecticut.

Greenberg is known for flying red flags over public companies. According to a Harvard Business School study, Greenberg has reported alleged inappropriate accounting on different companies more than any other reporter.

He started an investigation into the software company AremisSoft, which was later sued by the Securities and Exchange Commission (SEC) for fraud around 4 October 2001. According to the SEC, Its former chief executive, Roys Poyiadjis, later consented to disgorge around $200 million "of unlawful profit from his trading in AremisSoft stock -- among the largest recoveries the SEC has obtained from an individual."

Later he did an investigation into A.C.L.N., a Belgian company whose ships transported cars to Africa from Europe. It was later sued by the SEC on 8 October 2002 and delisted by the New York Stock Exchange. In 2004 the company agreed to disgorge around $27.6 million, which as to be returned to investors. The company has subsequently disappeared.

Then he exposed accounting questions about MBIA, the bond insurer. Much of his focus was on the controversial retroactive reinsurance transaction involving the Allegheny Health, Education and Research Foundation (AHERF), which was the focus of separate investigations by the SEC, the New York Attorney General and the New York State Insurance Department. The company settled the case by paying $75 million in penalties and disgorgement.

In February 2006, Greenberg and other journalists were served subpoenas from the Securities and Exchange Commission demanding records of phone and e-mail communications with sources for an investigation of which the journalists were not the target. In a rare public reprimand, SEC chairman Christopher Cox said that he was not consulted prior to the issuance of the subpoenas by the SEC's San Francisco office, and that the "subpoena to a journalist which seeks to compel production of his or her notes and records of conversations with sources is highly unusual."

The subpoenas were harshly criticized by the media and by First Amendment groups. In an editorial, The Wall Street Journal said that "SEC scolds are harassing journalists who report market-moving facts based on their daily digging." The newspaper said "the journalists are suspected of having sources who tell them things that they then share with their readers or listeners. Where we come from this is called reporting, or providing facts to investors who can then make more informed decisions." Shortly after the subpoenas were disclosed, the SEC reversed course and said it would not enforce them. The investigation concerned a research firm called Gradient Analytics, which was subsequently dropped.