[1] Olsen was once a member of the Homeland Security Advisory Council,[2] but resigned on July 18, 2018, over immigration decisions to separate families.

[6] While away from Harvard, Olsen worked as a Summer Associate at the Schwalb Donnenfeld, Bray & Silbert law firm in Washington D.C in 1986.

[8] In 2003, he was appointed Deputy Chief for the Organized Crime and Narcotics Trafficking Section within the U.S. Attorney's Office and worked as a Special Counsel to FBI Director Robert Mueller from 2004 to 2005.

[10] In 2005, the United States Attorney for the District of Columbia, Kenneth L. Wainstein, appointed Olsen as the Chief of the newly created National Security Division of the U.S. Attorney's Office, where he headed an eleven-member division that prosecuted suspected terrorists, and those who are suspected of illegal arms and human trafficking.

"[14] He voiced his support and helped craft legislation for the 2008 expansion of the Foreign Intelligence Surveillance Act[15] and was in charge of coordinating other FISA-related litigation.

[7] In the aftermath of President Barack Obama signing Executive Order 13492, creating the Guantanamo Review Task Force to issue the closure of the Guantanamo Bay detention camp, United States Attorney General Eric Holder appointed Olsen as the executive director of the Task Force.

[16] After the task force released their final report in 2010, regarding the recommendations and evaluation of which captives to hold indefinitely and which prisoners should be transferred to their home country's, the Obama administration ultimately decided to continue the incarceration of all of the detainees at Guantanamo.

[20] On May 26, 2021, President Joe Biden announced Olsen as his nominee to serve as the Assistant Attorney General for the National Security Division;[21][22] his nomination was sent to the Senate the following day.

[28] On July 1, 2011, President Barack Obama nominated Olsen to replace Michael Leiter as Director of the National Counterterrorism Center.