Early career

Shannon subsequently joined the United States Foreign Service as a Foreign Service Officer. He was the Consular/Political Rotational Officer at the U.S. Embassy in Guatemala City, Guatemala from 1984 to 1986; Country Officer for Cameroon, Gabon and São Tomé and Príncipe from 1987 to 1989; and the Special Assistant to the Ambassador at the U.S. Embassy in Brasília, Brazil, from 1989 to 1992.^[7]

He served as the Regional Labor Attaché at the U.S. Consulate-General in Johannesburg, South Africa, from 1992 to 1996; as Political Counselor at the U.S. Embassy in Caracas, Venezuela, from 1996 to 1999; and as the Director for Inter-American Affairs at the National Security Council (NSC) from 1999 to 2000.^[7]

Shannon was appointed as U.S. Deputy Permanent Representative to the Organization of American States (OAS) from 2000 to 2001, serving under Luis J. Lauredo. From 2001 to 2002, Shannon was the Director of Andean Affairs at the Bureau of Western Hemisphere Affairs, Department of State. He served as Deputy Assistant Secretary of Western Hemisphere Affairs from 2002 to 2003.^[7]

Shannon then served as Special Assistant to the President and Senior Director for Western Hemisphere Affairs at the NSC from 2003 to 2005. In October 2005, he became Assistant Secretary of State for Western Hemisphere Affairs and served in that role until November 2009, when he was replaced by Arturo Valenzuela. He was appointed as United States Ambassador to Brazil in February 2010 and served there until September 2013. For a period from July 2011 to September 2011, he was concurrently the acting Under Secretary of State for Political Affairs at the Department of State.^[7] In 2012, the Senate granted Shannon the rank of Career Ambassador in the U.S. Foreign Service.^[9]

State Department Counselor

In December 2013, Shannon was appointed as Counselor of the United States Department of State, only the second Foreign Service Officer ever appointed to that office. In early 2015, President Obama issued an executive order levying sanctions against seven Venezuelan officials; the order also called Venezuela a threat to the United States. President Nicolás Maduro accused the United States government of planning to invade Venezuela.

In April 2015, The New York Times reported that Shannon was in Venezuela, and was supposedly going to meet with President Maduro. Shannon was supposedly there to deliver a message to Maduro from the United States government. The State Department later issued a statement saying that the Venezuelan government had invited the United States government to send a diplomat to Caracas to meet with Maduro before the Summit of the Americas meeting. The Venezuelan Foreign Ministry then issued a statement saying that Shannon had met with the Venezuelan Foreign Minister. The Foreign Ministry later confirmed that Shannon "brought a message from his government."^[10] Shannon retained his position as Counselor until February 12, 2016, when he was appointed as Under Secretary of State for Political Affairs for the last year of Barack Obama's presidency.^{[7][11][12][13]}

Acting Secretary of State

After Donald Trump was inaugurated 45th President of the United States on January 20, 2017, Shannon became Acting United States Secretary of State. He remained in office for only 12 days, when on February 1, 2017, the Senate confirmed Rex Tillerson, President Trump's nominee, as Secretary of State.^[8][14]

While Shannon was acting Secretary of State, President Trump dismissed many prominent and senior State Department officials, resulting in the firing or forced resignations of over a dozen career diplomats,^[15] leaving a majority of senior career positions at the Department vacant.^[16] Most notably, Thomas Countryman, acting Undersecretary for Arms Control and International Security, was on his way to Rome for an international meeting on nuclear weapons when he discovered that he had been summarily removed from his position. Without leaving the airport, he turned around and got on the first flight back to Washington.^[17]

Under Secretary for Political Affairs

On February 2, 2016, the Senate confirmed Shannon as Under Secretary of State for Political Affairs.^[2]

In June 2016, Shannon complained about the Government of China's claim of domination in the South China Sea, calling it "madness." On June 29, 2016, he met with India Foreign Secretary Jaishankar, and expressed support for the Indian government, calling it America's "anchor" in the Asia-Pacific region.^[18]

On February 1, 2018, Shannon announced his plans for retirement, saying he would stay in the job until a replacement is named. He told the Associated Press the retirement was for personal rather than political reasons, noting his age and the recent death of his mother.^[19]

In 2020, Shannon, along with over 130 other former Republican national security officials, signed a statement that asserted that President Trump was unfit to serve another term, and "To that end, we are firmly convinced that it is in the best interest of our nation that Vice President Joe Biden be elected as the next President of the United States, and we will vote for him."^[20]