Sorensen was born in Nebraska, the son of Christian A. Sorensen, a Danish American and the future attorney general of Nebraska, and Annis Chaikin, who was of Russian Jewish descent. He graduated from Lincoln High School in 1945. He earned a bachelor's degree at the University of Nebraska in Lincoln and attended law school at the same university, graduating first in his class.

Coauthorship of Profiles in Courage (1956)

At the age of 27, Sorensen had an important role in researching and drafting Kennedy’s Pulitzer Prize—winning book Profiles in Courage, prompting some controversy over the book's authorship. In May 2008, Sorensen clarified in his autobiography, Counselor how he collabarated with Kennedy on the book: "While in Washington, I received from Florida almost daily instructions and requests by letter and telephone - books to send, memoranda to draft, sources to check, materials to assemble, and Dictaphone drafts or revisions of early chapters." Sorensen, p. 146) Sorensen wrote that Kennedy "worked particularly hard and long on the first and last chapters, setting the tone and philosophy of the book" and that "I did a first draft of most chapters" and "helped choose the words of many of its sentences". JFK "publicly acknowledged in his introduction to the book my extensive role in its composition" (p.147)Sorensen claimed that in May 1957, Kennedy "unexpectedly and generously offered, and I happily accepted, a sum to be spread over several years, that I regarded as more than fair" for his work on the book. Indeed, this supported a long-standing recognition of the collabarative effort that Kennedy and Sorensen had developed since 1953. (Ibid)

In December 1957, syndicated columnist Drew Pearson, interviewed on TV by Mike Wallace, said, “Jack Kennedy is . . . the only man in history that I know who won a Pulitzer prize on a book which was ghostwritten for him.” Kennedy demanded a retraction. After Kennedy provided handwritten notes and Sorensen signed an affidavit attesting to Kennedy's authorship, Pearson acceded. Historian Herbert Parmet went on to analyze the text of Profiles in Courage, writing in his book The Struggles of John F. Kennedy (1980) that although Kennedy did oversee the production and provided for the direction and message of the book, it was clearly Sorensen who provided most of the work that went into the end product.

Such accusations had dogged Kennedy ever since his undergraduate thesis "Why England Slept" was accused by some, of being ghostwritten. Kennedy addressed the charges against Pearson, a popular columnist, although widely acknowledged as a muckraker, by some, saying to Sorensen, "We might as well quit if we let this stand. This challenges my ability to write the book, my honesty in signing it, and my integrity in accepting the Pulitzer Prize." (Sorensen, p. 149)

See also Profiles in Courage, section: Authorship controversy.

Sorensen was President Kennedy's Special Counsel & Adviser, and primary speechwriter, the role for which he is best remembered today. His help with the new president's inaugural address exhorted listeners to "Ask not what your country can do for you; ask what you can do for your country." This call to service is the phrase still most closely associated with the Kennedy administration. Although Sorensen played an important part in the composition of the Inaugural Address, "the speech and its famous turn of phrase that everyone remembers was," Sorensen firmly states (counter to what the majority of authors, journalists and other media sources have claimed), "written by Kennedy himself."

In the early months of the administration the scope of Sorensen's responsibilities lay within the domestic agenda; however, after the Bay of Pigs debacle Kennedy asked Sorensen to take part in foreign policy discussions as well. During the Cuban Missile Crisis Sorensen served as a member of ExComm and was named by Secretary of Defense Robert S. McNamara as one of the "true inner circle" members who advised the president, the others being Attorney General Robert F. Kennedy, National Security Adviser McGeorge Bundy, Secretary of State Dean Rusk, General Maxwell D. Taylor (the Chairman of the Joint Chiefs), former Ambassador to the Soviet Union Llewellyn Thompson and McNamara himself. Sorensen played a critical role in drafting Kennedy's correspondence with Nikita Khrushchev and worked on Kennedy's first address to the nation about the crisis on October 22.

Sorensen was devastated by Kennedy's assassination, which he called "the most deeply traumatic experience of my life...I had never considered a future without him." He submitted a letter of resignation to President Johnson the day after the assassination but was persuaded to stay through the transition. Sorensen drafted Johnson's first address to Congress as well as the 1964 State of the Union. He officially resigned February 29, 1964, and was the first member of the Kennedy Administration to do so.

Prior to his resignation, Sorensen stated his intent to write Kennedy's biography, calling it "the book that President Kennedy had intended to write with my help after his second term." He was not the only Kennedy aide to turn to writing; historian and Special Assistant Arthur M. Schlesinger, Jr. wrote his Pulitzer-winning memoir John F. Kennedy in the White House during the same time span. Sorensen's biography Kennedy was published in 1965 and became an international bestseller.

Sorensen later joined the prominent U.S. law firm of Paul, Weiss, Rifkind, Wharton & Garrison LLP, while still staying involved in politics. He was an important part number of Democratic campaigns and was a key adviser to Robert F. Kennedy in Kennedy's 1968 presidential campaign. Over the past four decades, Sorensen has led a prominent career as an international lawyer, advising governments around the world, as well as major international corporations.

In 1970, Sorensen ran as the party leaders' designee for the Democratic nomination for U.S. Senator from New York, but was challenged in the primary election by Richard Ottinger, Paul O'Dwyer and Max McCarthy, and came in third. In 1977, Jimmy Carter nominated him as Director of Central Intelligence, but the nomination was withdrawn before a Senate vote. Sorensen’s help in explaining Ted Kennedy’s Chappaquiddick incident was cited as one factor in Senate opposition to his nomination as CIA Director.

Sorensen was the national co-chairman for Gary Hart for the presidential election of 1984 and made several appearances on his behalf.

In addition to his successful career as a lawyer, Sorensen has also been a frequent spokesman for liberal ideals and ideas, writing op-eds and delivering speeches on both domestic and international subjects. For several years in the 1960s, he was an editor at the Saturday Review.

He has been affiliated with a number of institutions, including the Council on Foreign Relations, The Century Foundation, Princeton University, and the Institute of Politics at Harvard's John F. Kennedy School of Government. Sorensen is a board member of the International Center for Transitional Justice and an Advisory Board member of the Partnership for a Secure America, a not-for-profit organization dedicated to recreating the bipartisan center in American national security and foreign policy. He is also chair of the advisory board to the International Center for Ethics, Justice and Public Life at Brandeis University. Sorensen has also attended meetings of the Judson Welliver Society, a bipartisan social club composed of former presidential speechwriters.

In 2007, a model Democratic presidential nomination acceptance speech written by Sorensen was published in the Washington Monthly. The magazine had solicited him to write the speech that he would most want the 2008 Democratic nominee to give at the 2008 Democratic National Convention, without regard to the identity of the nominee.

On March 9, 2007 he spoke at an event with then-Senator Obama at New York City's Grand Hyatt Hotel and officially endorsed Barack Obama for the presidential election in 2008. Very active in his campaign, Sorensen spoke (early-on and) frequently about the similarities between both Senator Barack Obama's and Senator John F. Kennedy's presidential campaigns. He also provided some assistance with President Obama's 2009 Inaugural Address.

Sorensen serves on the Advisory Board of the National Security Network.

He is married to Gillian Sorensen of the United Nations Foundation. He has three sons: Eric, Stephen, and Philip and a daughter, Juliet Sorensen.

In the 1998 miniseries From the Earth to the Moon, Sorensen was played by Jack Gilpin; in the 2000 film Thirteen Days, although he was played by Tim Kelleher, it is widely believed that the lead role played by Kevin Costner was modeled after Sorensen himself.

On February 25, 2010, he received the National Medal of Humanities for 2009 in a ceremony in the East Room of the White House. He was awarded the medal for: "Advancing our understanding of modern American politics. As a speechwriter and advisor to President Kennedy, he helped craft messages and policies, and later gave us a window into the people and events that made history."