"I have come to learn during the confirmation review process that I failed to pay FICA and federal and state unemployment taxes on a household employee for the years 2000-2004," Mulvaney said in a questionnaire submitted to the Senate Budget Committee, which is vetting his nomination.
Mulvaney also states in his questionnaire that he paid the federal taxes "upon discovery of that shortfall."
"The amount in question for federal FICA and unemployment was $15,583.60, exclusive of penalties and interest, which are not yet determined," Mulvaney's answer reads.
The issue could be raised in Mulvaney's confirmation hearing that's set for Tuesday Jan. 24. Similar tax compliance issues have thwarted some previous administrations' cabinet nominees from being confirmed.
Mulvaney’s office declined to comment, referring comment to the Trump transition team, which is sticking by Rep. Mulvaney.
“The fact of the matter is that nobody is more qualified and more prepared to fight to rein in Washington spending and fight for taxpayers than Mick Mulvaney," "Congressman Mulvaney raised the issue surrounding the care of his premature triplets immediately upon being tapped for this position, and has taken the appropriate follow-up measures."
The South Carolina congressman is the second of Trump's cabinet nominees to admit to employment issues.
"We did the best that we thought we could do in order to verify the legality of the employment and it turned out that was incorrect," Ross said.
ABC News' John Parkinson contributed to this report.