By Damian Paletta 

The Obama administration's point man in the effort to deter U.S. companies from restructuring overseas to avoid taxes is a career government economist known for his loosened neckties, Pez dispensers and sweeping grasp of the U.S. tax code.

Mark J. Mazur, the Treasury's assistant secretary for tax policy, is helping assemble a menu of administrative options for Treasury Secretary Jacob Lew that would aim to upend the economic benefits of the so-called inversions that occur when a cross-border acquisition allows a U.S. firm to avoid paying U.S. taxes.

The Treasury Department has said it is acting in response to a wave of recent merger proposals that would use an inversion blueprint. These include Covidien PLC's decision to sell itself to Medtronic Inc., and the move by Shire PLC to sell itself to AbbVie Inc.

Given Mr. Mazur's power--a number of multibillion-dollar mergers this year could hinge on his recommendations--little is known about him outside the halls of Congress and the White House. But a review of his record and interviews with colleagues reveal him to be a careful economist with a focus on what he perceives as "fairness" in the tax code.

Mr. Mazur, 58 years old, graduated from Michigan State University and received a Ph.D. in business from Stanford University. He worked as a tax accountant at General Motors Co., and then taught public finance for four years at Carnegie Mellon University. He then worked for Congress's Joint Committee on Taxation and, later, in midlevel roles in the Clinton White House's economic team, as well as the Energy Department and the Internal Revenue Service.

While at the IRS, he served as director of research, analysis, and statistics of income, a senior-level post considered one of the federal government's most important in terms of analysis of the tax code's interactions with Americans and businesses.

He joined the Treasury in 2009, working as deputy assistant secretary for tax analysis. He was nominated to his current role in 2011 and has worked on range of issues, including the implementation of the Affordable Care Act and the IRS's review of the tax-exempt status of nonprofits.

"Whether people are planning for retirement, running a business, deciding to get married, or saving for college, the work we do has an impact on just about every American household," said Mr. Mazur. "What I always think about is how tax affects ordinary Americans, and I try to ensure that our nation's tax policies are fair, effective, and efficient."

Colleagues described him as even-keeled, with an endless understanding of the tax code.

"He really knows the data, so nobody can pull a fast one on Mark in terms of fact-based assertions," said Stephen Shay, a professor at Harvard Law School who worked with Mr. Mazur at the Treasury until 2011. "He's got as broad an economic knowledge of tax and related areas as anyone."

In fact, outside groups have bemoaned their inability to convince Mr. Mazur of their point of view. Robert McIntyre, director of the liberal-leaning Citizens for Tax Justice, recalled meeting with Mr. Mazur and complaining about how the White House had backed off an unspecified tax change.

"Sometimes we will go down and talk to him and say 'you guys had a pretty good proposal on this thing, and now you are backing off on it,' " Mr. McIntyre said, with the hopes that Mr. Mazur would reconsider. " 'That's not what we are doing now,' he would say."

Many Treasury officials, given their influence over a range of industries, find themselves followed by a flock of analysts or reporters. Mr. Mazur--not so much. Colleagues said they would joke with him about his affinity for the New York Giants and New York Mets, and he has a number of Pez dispensers arranged around his Treasury office--gifts from family and friends.

But few outside of government know much about him or study his every move. In late 2013, he was interviewed about his views on tax policy by Rep. Bill Pascrell (D., N.J.) in a video the congressman produced. The segment had only 14 views on YouTube as of early Wednesday.

With his customary loosened tie and unbuttoned collar, Mr. Mazur acknowledged during the segment that many Americans feel the tax code is unfair and benefits primarily the wealthy, but he said that can be a misconception.

"If you look at the data, the data seem to indicate that higher income people tend to pay a larger share of their income--on average--than lower income people," he said in response to a question.

But he also said many companies and people were looking for ways to take advantage of the tax code. He said "you have corporate interests and high-income individuals looking for ways to shelter or lower their tax bills."

He also called for changes to the tax code that would effectively require hedge-fund managers to pay higher tax rates on their income, saying they benefit from what he called an unfair "loophole" which allows income earned by hedge fund and private equity managers--a share of profits known as carried interest--to be taxed at a lower rate than regular income. The White House believes this income should be taxed at the same rate as regular income.

"It doesn't benefit generally the economy as a whole," he said. It "certainly benefits the individuals who take advantage of it."

Mr. Mazur is so under-the-radar that Grover Norquist, president of Americans for Tax Reform, said he had never heard of him. Mr. Norquist, in an interview, said he doubted people like Mr. Mazur would have much say in the final White House proposal on inversions, believing it instead would be a political decision made by the White House to score political points.

"All [Mr. Mazur] can do is say 'here are the things you could claim are executive powers that nobody else in the world has ever thought were,' " Mr. Norquist said.

Mr. Mazur's seat at the table was on full display on Tuesday, when the White House released a list of economic advisers who briefed President Barack Obama and Vice President Joe Biden. Mr. Mazur's name was on the list, which many in Washington, Wall Street, and even in Europe read as a signal that a decision on how to tackle inversions was near.

Meanwhile, lawyers, bankers, and chief executives will be hanging on any utterance or clue from Mr. Mazur.

"He has a very disciplined and careful mind, and that's what really matters," said Edward Kleinbard, former chief of staff of Congress's Joint Committee on Taxation.

Write to Damian Paletta at damian.paletta@wsj.com

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