In 2003, Olsen founded Klafter & Olsen. His work involved defending companies in product liability cases. In October 2007, Maryland governor Martin O'Malley named Olsen to his Health Care Commission. In 2021, after Olsen's involvement in litigation surrounding the 2020 presidential election became apparent, the firm changed its name to Klafter Lesser and disassociated with him completely.
After the 2020 presidential election, Olsen began involving himself with lawsuits that falsely contended Joe Biden's victory. He advised Texas attorney general Ken Paxton on his lawsuit against Pennsylvania. According to emails obtained by House Committee on Oversight and Government Reform investigators, Olsen attempted to contact Jeffrey A. Rosen, the acting U.S. attorney general, about the lawsuit. He spoke with Jeff Wall, the acting U.S. solicitor general, asserting that president Donald Trump had directed him to speak to Rosen but that he could not reach Rosen. After Rosen's chief of staff told him that Rosen was busy, Olsen drove to Washington, D.C., "in the hopes of meeting" with him. Rosen was not convinced by Olsen's arguments and challenged him on the case's merit. The lawyer William J. Olson encouraged Trump to hire Olsen.
In August 2021, the House Select Committee to Investigate the January 6th Attack on the United States Capitol sent requests to government agencies inquiring about Olsen's activities. According to a statement accompanying a subpoena sent in March 2022, the committee alleged that Olsen had prepared a draft executive order to direct the Department of Justice to "take voter action" and had spoken repeatedly with Trump on January 6. Mike Lindell, the businessman who founded My Pillow, established a network of election denialists that included Olsen. In April 2022, prior to that year's Arizona gubernatorial election, Olsen filed a lawsuit on behalf of the former television presenter Kari Lake and the politician Mark Finchem challenging Arizona's use of electronic voting machines. After Katie Hobbs defeated Lake in the election, Lake filed a lawsuit in state court to overturn the results. The 65 Project filed a complaint against Olsen in Maryland over his election litigation in August 2022 and in February 2023. The Arizona Supreme Court sanctioned Lake, the lawyer Bryan Blehm, and Olsen in May over false statements in their lawsuit. The State Bar of Arizona began investigating two charges brought against Olsen in November.
In October 2025, The Wall Street Journal reported that Trump had requested Olsen examine the results of the 2020 presidential election as a special government employee in that past month, and that he had begun requesting information from intelligence agencies. Olsen had additionally told others he sought to remove government employees he perceived were disloyal to Trump. Speaking to Steve Bannon on War Room (2019–present) in January 2026, Ed Martin, the United States pardon attorney and the head of the Weaponization Working Group, claimed that he was working with Olsen—describing him as a White House lawyer—and "a couple of others" on election integrity, indicating that their work would cross with the Federal Bureau of Investigation's investigation into the 2020 election in Georgia. According to an affidavit released after the Federal Bureau of Investigation raided the election center in Fulton County, Georgia in January 2026, Olsen initiated the investigation with a criminal referral.
According to Politico, Olsen's work involved compiling a report summarizing an inquiry into whether Biden legitimately won the 2020 election. In investigating the election, Olsen reviewed sensitive compartmented intelligence programs and frequently contacted Trump to access information quickly. The Central Intelligence Agency publicly confirmed that it had coordinated with Olsen; since he became a special government employee, Olsen traveled several times to the headquarters of the Office of the Director of National Intelligence, the Central Intelligence Agency, and the National Security Agency, and he was in contact with the Department of Justice and the Federal Bureau of Investigation. Olsen was particularly in close contact with Andrew Bailey, the deputy director of the Federal Bureau of Investigation, and Tim Kosiba, the deputy director of the National Security Agency, and had encouraged Trump to nominate Kosiba to his position. Virginia senator Mark Warner, the vice chair of the Senate Select Committee on Intelligence, criticized Tulsi Gabbard, the director of national intelligence, for permitting Olsen's appointment.