Ending a 13-year run as a Federal Trade Commission litigator, Matt Reilly is taking a job with Simpson Thacher & Bartlett LLP as a partner in its Washington office.

Since September 2007, Reilly has been assistant director of the FTC's Mergers IV unit, which reviews mergers in the hospital, retail and consumer products businesses and handles the litigation when the commission decides any deals pose a threat to competition. He was a key player as the FTC pushed to reinvigorate merger litigation, which had been moribund after the agency failed to persuade a federal judge to halt Arch Coal Inc.'s purchase of a rival coal mining operation in 2004.

Three years later, the commission revived its litigation efforts when it attempted to block Western Refining Inc.'s $1.4 billion purchase of Giant Industries Inc. Since then, the commission has filed more than a dozen merger challenges. Reilly calls the revival of antitrust litigation "the most exciting time of my career."

Now with three children under age 6, Reilly, 46, says the better pay of a private law firm was a major factor in leaving the FTC. As for joining Simpson Thacher, Reilly says it was the best fit of all the firms he considered. He also was incredibly impressed with the high quality of the firm's other partners and associates. "It seemed everyone was the best in their area of service."

At the FTC, Reilly most recently led the successful in-house litigation to stop ProMedica Health System Inc.'s takeover of St. Luke's Hospital in Toledo, Ohio. He also was the lead attorney in the commission's 2007 federal court case against Whole Foods Market Inc.'s purchase of Wild Oats Markets Inc., which resulted in a ruling supporting the FTC after a long legal battle.

Had the case gone to trial, Reilly would have been the FTC's primary trial lawyer in the commission's bid to halt the merger of blood plasma competitors CSL Ltd. and Talecris Biotherapeutics Inc. in 2008. That would have pitted Reilly against the man who is now his boss, Kevin Arquit, head of Simpson Thacher's antitrust practice. CSL, however, dropped its plan to acquire Talecris when faced with the FTC challenge.

Another milestone in the FTC's revival was the clear likelihood of winning a preliminary injunction against Inova Health System Foundation's 2008 attempt to buy a rival hospital in northern Virginia, which prompted the parties to drop the deal and ended what Reilly calls the FTC's "long, rich losing streak" in hospital merger challenges.

The FTC's courtroom resurgence preceded a similar revival at the Department of Justice. In its 2008 effort to break up the consummated merger of military semiconductor makers Microsemi Corp. and Semicoa Corp., the DOJ ended a merger litigation hiatus that began with its 2004 defeat in the case challenging the merger of Oracle Corp. and PeopleSoft Inc.

Since 2007 there has been an "amazing amount of litigation after a three-year lull," Reilly says. "Both agencies revisited what they could do in court after Arch Coal and Oracle."

Reilly says the regulators had to regroup after federal judges rejected government cases that depended greatly on customer testimony. "It took some time to find the right cases," he says.

Two major developments during Reilly's tenure also added to the FTC's leverage in court when bringing merger challenges. One was a 2008 rewrite of the agency's rules that sped up in-house merger trials, making them a more practical option for halting a merger. The other was rulings in both the Whole Foods case and the 2009 case against CCC Information Services Inc.'s attempt to buy Mitchell International Inc., in which judges declared that the FTC faces a lower bar winning preliminary injunctions than other government agencies do. Reilly also credits his former colleagues on the Mergers IV team, which he calls the "pride of the FTC."

Hogan Lovells US LLP partner J. Robbie Robertson, a former chief trial counsel in the FTC's bureau of competition, worked with Reilly on a number of cases. "He's one of the few people in the business who are not only excellent lawyers, but excellent trial lawyers," Robertson says of Reilly. "People at the FTC would transfer or move across divisions to work with Matt."

Reilly, a 1995 graduate of the Georgetown Law Center, majored in economics as an undergraduate and finished at the top of his class when he earned an M.A. in economics from Duke University. At Rogers & Wells, where he worked with Arquit, he won a spot on the team representing Cardinal Health Inc. against the FTC's 1998 bid to stop the company's acquisition of Bergen Brunswig Corp.

"I fell in love with antitrust litigation at that point -- the excitement, challenge, a clear outcome of winners and losers, and the prospect of playing a major role in seminal antitrust litigations and case decisions," he says. "At that point I had two goals -- to continue to work on antitrust litigation and to be the one at the podium doing the opening, closing and crosses."

From 2007 on, Reilly's been doing just that.