In January 2022, the Internal Revenue Service was cracking down on a tax dodge from the agency’s “dirty dozen” list of abusive shelters. To fight back, promoters of the scheme turned to the lobbyist Ken Kies.

In a conference call with lawyers and financial advisers, Mr. Kies outlined plans to fight the I.R.S., including by capitalizing on his close relationship with a top agency official, according to a recording of the call obtained by The New York Times.

Now Mr. Kies has become the Treasury Department’s top tax policy official. The former veteran lobbyist, who has worked for some of America’s biggest companies, was confirmed by the Senate last month to serve as Treasury’s assistant secretary for tax policy.

It is not uncommon in President Trump’s Washington for lobbyists or other interested parties to get high-level positions at agencies where they once sought access on behalf of corporate clients. But Mr. Kies is not just any lobbyist. For decades, he has played an instrumental role in enabling some of the most lucrative and most important tax avoidance strategies used by multinational companies and the wealthiest Americans.

When the Clinton administration sought to stem the tide of companies shifting trillions of dollars of profits into offshore havens, Mr. Kies led the effort on behalf of a coalition of businesses to kill the regulation. In the George W. Bush administration, Mr. Kies successfully pushed for legislation to make such offshore tax dodges even easier to execute. During the Obama administration, he fended off another attempted crackdown on those strategies.

In 2017, as part of a sweeping package of tax cuts signed by Mr. Trump, Mr. Kies lobbied for a new tax break that provides a 20 percent deduction to certain businesses, which overwhelmingly benefits the richest Americans. And most recently, he advised the Trump Organization on a dispute with the I.R.S.