Under Christine Varney, the Justice Department approved a number of high-profile, controversial deals, including NBC’s merger with Comcast. (Carolyn Kaster/AP)

President Obama’s top antitrust cop, Christine Varney, is stepping down from her post next month, the Justice Department announced Wednesday.

Varney came into her job with high expectations that she would launch landmark cases against increasingly dominant firms such as Google. But she leaves for a senior position at the prestigious law firm Cravath, Swaine & Moore amid disappointment from some antitrust watchers and consumer advocates who say the Justice Department was too soft on industry giants during her tenure.

Under Varney, the Justice Department approved a number of high-profile, controversial deals, including the marriage of Ticketmaster and LiveNation, Google’s acquisition of a powerful travel software firm, and NBC’s merger with Comcast. In each of these instances, Varney attached limits on how the companies could behave to address worries that the deals would hurt competition.

Bert Foer, president of the American Antitrust Institute, said he was giving Varney’s tenure a “B-minus” grade for now. It will take years, he said, to know whether the strings she attached to these big mergers will work in curbing bad behavior at dominant companies.

“You had a series of very important mergers in which she probably took more action than the previous administration would have, but the mergers were allowed and the remedies are controversial and very difficult to evaluate,” Foer said.

Varney’s arrival at the Justice Department in 2009 was highly anticipated by consumer advocates who thought the Bush administration’s antitrust attorneys had been too easy on big firms. Varney gave signals that she would be a much tougher cop.

In May 2009, Varney said in a speech: “It is time for the antitrust division to step forward again. We must change course and take a new tack.”

In 2008, Varney said at a discussion sponsored by the American Antitrust Institute that Microsoft was “so last century” and that Google could be a potential problem. The comment, which came months before President Obama nominated her to head the Justice Department’s antitrust division, spurred speculation that she could be the next Joel Klein, the aggressive antitrust enforcer under President Clinton who went after Microsoft in an epic government case.

The Justice Department examined Google while deciding whether the company could acquire the travel software firm ITA. Varney eventually approved the deal while adding some restrictions on Google’s behavior.

But it was ultimately the Federal Trade Commission, rather than the Justice Department, that won the task of launching a broader investigation into the search giant.

In reviewing some blockbuster mergers, Varney often pursued outcomes that did not involve litigation, instead applying solutions that forced companies to spin off certain units or follow certain rules. These conditions, Varney said, assuaged the Justice Department’s concerns.

The Justice Department’s antitrust division did, however, go to court in some more low-profile cases, suing Blue Cross Blue Shield of Michigan for making agreements with hospitals that Varney said hurt competition and consumers. The department also went to court to oppose an acquisition by Dean Foods, the country largest dairy processor.

“The problem is, everyone’s looking for the headline-grabbing case,” said David Balto, an antitrust expert at the Center for American Progress. “It’s a lot of these smaller, more quiet cases that are every bit as significant.”

“There is no doubt that her tireless work helped protect consumers and businesses from anticompetitive conduct and preserved competition in America’s economy,” Attorney General Eric H. Holder Jr. said in a statement Wednesday.

Cravath, which is known for its antitrust practice, represented United Airlines in its merger with Continental. Varney’s division at the Justice Department greenlighted the deal last August.

The firm’s average profits per partner in 2010 were $3.17 million, the fifth highest in the country, according to an annual ranking by American Lawyer.