"[It was] the most deeply traumatic experience of my life. ... I had never considered a future without him," he said of losing Kennedy.
"Sometimes," Sorensen he said to an interviewer in 2006, "I still dream about him."
In his 2008 biography, "Counselor," Sorensen calls his 11 years with Kennedy "the cornerstone of my professional life; and the cornerstone of our relationship was mutual trust. JFK brought me into his inner circle, confiding in me secrets that -- had I discussed them with others -- might have done serious harm to his political career, his public image, or perhaps his marriage."
"What's striking about Sorensen was not just his unique partnership with JFK," said ABC News' John Hendren, "but also his staggering humility and loyalty -- a loyalty that has shown him quietly abstaining from joining in the chatter on the most controversial topics related to JFK."
After resigning from the Johnson administration, Sorenson began his legal career at Paul, Weiss, Rifkind, Wharton & Garrison LLP, which saw him through the next four decades.
He also dabbled on the fringes of politics, advising Robert Kennedy in his 1968 presidential campaign and running for the Democratic nomination for the Senate in New York in the early 1970s.
In 2008 he threw his support behind Barack Obama's Presidential bid, often drawing comparisons between Obama's and Kennedy's presidential campaigns.
Sorenson wrote four books throughout his life, including his 2008 autobiography "Counselor," "Decision Making in the White House" in 1963, "Kennedy" in 1965 and "The Kennedy Legacy" in 1969.
In addition to his wife Gillian, Sorenson is survived by his sons Eric, Stephen, and Philip, and his daughter, Juliet. The Associated Press contributed to this report.