WASHINGTON—A Treasury Department official deeply involved in implementing the new tax law left the government unexpectedly this week.

Dana Trier, a retired New York attorney praised by fellow tax lawyers in both parties, was a deputy assistant secretary for tax policy, putting him near the center of administration decision-making about how to write the crucial and highly technical rules stemming from the new Tax Cuts and Jobs Act.

Accountants, tax lawyers and businesses have been watching his actions and speeches closely for clues on how the Trump Administration will enforce complex new deductions for pass-through businesses, restrictions on business interest deductions and other matters.

As recently as Tuesday, he spoke publicly about the stream of rules poised to come from the U.S. in the next year. The Treasury needs to clarify vague sections of the law, which went from draft to reality in less than two months.

Mr. Trier’s departure was confirmed by Treasury spokesman Tony Sayegh on Friday. Mr. Trier was part of a large team that will continue to work without him. Treasury officials expect to release a calculator for paycheck withholding next week.