WASHINGTON — A top official under Deputy Attorney General Lisa O. Monaco is expected to step down this summer, as her office juggles numerous high-profile national security and criminal issues, the Justice Department said on Thursday.

The official, John Carlin, occupies one of the most powerful and under-the-radar posts in the Justice Department. He advises Ms. Monaco and Attorney General Merrick B. Garland on major prosecutions, such as the Jan. 6 investigation, and other top departmental priorities, including oversight of the F.B.I., an initiative to curb corporate crime and task forces to crack down on cryptocurrency theft and those who help Russia evade sanctions.

Mr. Carlin, the principal associate deputy attorney general, is likely to return to private practice. Before joining the Biden administration, he helped lead the national security practice at Morrison & Foerster and focused on cybersecurity.

“I am grateful that John readily agreed to return to the Department of Justice on Day 1 of this administration, even though he always made it clear it would only be for a limited term,” Ms. Monaco said in a statement, adding that she expected he would return to government someday. “His wise counsel will be missed.”

Mr. Carlin played a critical role in the early days of the Justice Department under President Biden.

He told colleagues at the time that he intended to stay on for a year to help the new administration transition as it responded to a series of crises, including the Jan. 6 riot, the pandemic and Russian cyberattacks on the U.S. government that were discovered during the final months of the Trump era.

Mr. Carlin became the acting deputy attorney general once Mr. Biden was sworn in on Jan. 20, 2021, taking on day-to-day oversight of the Jan. 6 investigation and other key prosecutions.

He worked with the transition team, career officials and others to understand the effect that the previous administration had on a department that former President Donald J. Trump had considered a foe.

And once Ms. Monaco was confirmed, Mr. Carlin became her top deputy, managing many of the department’s daily functions and helping determine what issues to bring to her attention.

Mr. Carlin was also involved in policy issues such as cybersecurity and cyberthreats, and acted as a main point of contact between Ms. Monaco’s office and the nation’s federal prosecutors.

Mr. Carlin’s career in government is closely entwined with that of Ms. Monaco, who has been his mentor, boss and friend over the years.

Both served as federal prosecutors in the U.S. attorney’s office in Washington from 2001 to 2007. During that time, Ms. Monaco’s stature rose because of her service on the Enron task force, a group of lawyers who investigated and prosecuted one of the largest corporate frauds of that era.

Ms. Monaco went on to serve as chief of staff to Robert S. Mueller III when he was the director of the F.B.I., and Mr. Carlin worked as a special counselor to Mr. Mueller and then became his chief of staff. In addition to counterterrorism work, both of them became deeply involved in cybersecurity as it became a fast-growing national security threat.

When Ms. Monaco ran the Justice Department’s national security division in 2011, Mr. Carlin served as her deputy. When she left in 2013 for the White House to serve as President Barack Obama’s homeland security and counterterrorism adviser, he became the acting head of the national security division. In 2014, the Senate confirmed him to lead it.

Both Ms. Monaco and Mr. Carlin left the government in 2017, eventually landing at prestigious law firms in Washington and returning only after the Trump administration ended.