Johnson is currently a partner at the law firm Paul, Weiss, Rifkind, Wharton & Garrison, a member of the board of directors of MetLife, and a trustee of Columbia University.

[4] Among other notable family members, Norma's brother Kenneth C. Edelin was a physician and a defendant in a landmark case involving abortion rights.

Johnson began as an associate at Paul, Weiss, Rifkind, Wharton & Garrison in November 1984, then left in 1989 to serve as an assistant United States Attorney in the public corruption section at the Southern District of New York.

In a speech delivered at The Heritage Foundation in October 2011, Johnson warned against "over-militarizing" the U.S. government's approach to counterterrorism: "There is risk in permitting and expecting the U.S. military to extend its powerful reach into areas traditionally reserved for civilian law enforcement in this country.

[24] At the Oxford Union in November 2012, shortly before his resignation, Johnson delivered an address titled "The conflict against al Qaeda and its affiliates: how will it end?"

Peace must be regarded as the norm toward which the human race continually strives.The Oxford Union speech received widespread press attention,[25][26][27][28] and editorial acclaim as the first such statement coming from an Obama administration official.

[29] According to published reports, Johnson personally authored the legal opinion that provided the basis for U.S. special operations forces to go into Pakistan to kill Osama bin Laden.

[30] Johnson was nominated by President Barack Obama to be the fourth U.S. Secretary of Homeland Security in October 2013, and was subsequently confirmed by the United States Senate on December 16, 2013, by a 78–16 vote.

[35] After the flow of immigrant children to the United States, the Department of Homeland Security established three family residential centers, and they immediately became the focus of much controversy.

[36] The ACLU has compared them to Japanese internment camps and in July 2015 a U.S. District Court Judge in California ordered that the family residential centers comply with a 1997 settlement concerning the detention of children.

[36] During the summer and fall of 2014, Secretary Johnson oversaw the Department of Homeland Security's response to the ongoing Ebola crisis in West Africa.

[37] This response was chosen by the department over limiting travel visas to the United States, which Secretary Johnson contended would have been a mistake given the leadership position of the U.S. and likelihood of influencing other countries to take the same action.

[38] After the House of Representatives failed to act on Bill S. 744, Secretary Johnson and President Obama issued ten new executive actions on November 20, 2014, to address the 11 million undocumented individuals in the United States.

[41] After leaving office in January 2017, Johnson rejoined the law firm Paul, Weiss, Rifkind, Wharton & Garrison in New York City.

He is also now a frequent commentator on NBC's Meet the Press, CBS, MSNBC, CNN, NewsNation, Bloomberg Television, and other networks, and the author of numerous op-eds.

[54][55] In April 2020, Governor Phil Murphy appointed Johnson to represent New Jersey in the seven-state regional working group to develop a plan for reopening the economy following the COVID-19 crisis.

[56] In June 2020, Chief Judge of New York State Janet DiFiore, appointed Johnson as Special Advisor on Equal Justice in the courts.

In the report Johnson noted: “[I]n one form or another, multiple interviewees from all perspectives still complain about an under-resourced, over-burdened New York State court system, the dehumanizing effect it has on litigants, and the disparate impact of all this on people of color.

[59] In June 2023, the president of the New York State Bar Association asked Johnson to co-chair a task force to study the impact of the Supreme Court's decision in the Harvard and UNC cases on affirmative action.