Mark Meadows, the final White House chief of staff under President Donald J. Trump and a potentially key figure in inquiries related to Mr. Trump, has testified before a federal grand jury hearing evidence in the investigations being led by the special counsel’s office, according to two people briefed on the matter.

Mr. Meadows is a figure in both of the two distinct lines of inquiry being pursued by the special counsel appointed to oversee the Justice Department’s scrutiny of Mr. Trump, Jack Smith.

One inquiry is focused on Mr. Trump’s efforts to cling to power after losing the 2020 election, culminating in the attack by a pro-Trump mob on the Capitol during congressional certification of the Electoral College results on Jan. 6, 2021. The other is an investigation into Mr. Trump’s handling of hundreds of classified documents after he left office and whether he obstructed efforts to retrieve them.

It is not clear precisely when Mr. Meadows testified or if investigators questioned him about one or both of the cases.

For months, people in Mr. Trump’s orbit have been puzzled by and wary about the low profile kept by Mr. Meadows in the investigations. As reports surfaced of one witness after another going into the grand jury or to be interviewed by federal investigators, Mr. Meadows has kept largely out of sight, and some of Mr. Trump’s advisers believe he could be a significant witness in the inquiries.

Mr. Trump himself has at times asked aides questions about how Mr. Meadows is doing, according to a person familiar with the remarks.

Asked about the grand jury testimony, a lawyer for Mr. Meadows, George Terwilliger, said, “Without commenting on whether or not Mr. Meadows has testified before the grand jury or in any other proceeding, Mr. Meadows has maintained a commitment to tell the truth where he has a legal obligation to do so.”

Mr. Meadows was a polarizing figure at the White House among some of Mr. Trump’s aides, who saw him as a loose gatekeeper at best during a final year in which the former president moved aggressively to mold the government in his image.

Mr. Meadows was around for pivotal moments leading up to and after the 2020 election, as Mr. Trump plotted to try to stay in office and thwart Joseph R. Biden Jr. from being sworn in to succeed him. Some of them were described in hundreds of text messages that Mr. Meadows turned over to the House select committee that investigated the Jan. 6 attack at the Capitol before he decided to stop cooperating. Those texts served as a road map for House investigators.

But Mr. Meadows also has insight into efforts by the National Archives to retrieve roughly two dozen boxes of presidential material that officials had been told Mr. Trump took with him when he left the White House in January 2021. Mr. Meadows was one of Mr. Trump’s representatives to the archives, and he had some role in trying to discuss the matter with Mr. Trump, according to two people briefed on the matter.

Mr. Meadows is also now connected tangentially to a potentially vital piece of evidence that investigators uncovered in recent months: an audio recording of an interview that Mr. Trump gave to two people assisting Mr. Meadows in writing a memoir of his White House years.

Mr. Meadows did not attend the meeting, which took place in July 2021 at Mr. Trump’s club at Bedminster, N.J. During the meeting, Mr. Trump referred to a document he appeared to have in front of him and suggested that he should have declassified it but that he no longer could, since he was out of office.

That recording could undercut Mr. Trump’s claim that he believed he had declassified all material still held at his properties for months after he left office.