Kellen was a member of a prominent Jewish family in Berlin, the son of Ludwig Katzenellenbogen. He was a distant relative of Albert Einstein and a cousin of economist Albert Otto Hirschman.^[1] He studied law in Munich before fleeing Germany with his family in March 1933 at age 19 to escape Nazi persecution.^[2][3] After living in France, the Netherlands and Yugoslavia, Kellen traveled to the United States.^[2] He arrived in New York in 1935 and moved to Los Angeles, becoming an American citizen under the name Konrad Kellen.^[2] From 1941-1943, Kellen was the private secretary to author Thomas Mann.^[3]

During World War II, Kellen trained at Camp Ritchie where he trained to become one of the Ritchie Boys. In the U.S. Army, he worked in an intelligence unit in Europe,^[1] working in psychological warfare, and being awarded the Legion of Merit.^[2] After the war, Kellen remained in Germany as a political intelligence officer with the occupation forces^[4] as part of the denazification initiative,^[2] his duties including interviewing German soldiers to find out why they kept fighting for Hitler long after it was clear that their war was lost.^[1] Kellen also worked for Radio Free Europe, interviewing defectors from behind the Iron Curtain to study life under the Soviet regime.^[1]

In 1945, Marc Chagall's daughter approached Kellen in a Paris cafe and asked him to save a stack of Chagall's canvases from the chaos of postwar Europe by bringing them to the United States, which took Kellen over a month to accomplish.^[1][5] Kellen was given one of the Chagall paintings as compensation, which he sold in the 1950s.^[5]

In the early 1960s he worked at the Hudson Institute think tank with military strategist and futurist Herman Kahn.^[6] In the mid 1960s, Kellen joined the RAND Corporation.^[4] He was among the first to conclude, in 1965, that the Vietnam War was unwinnable.^[7] His opinion disagreed with the U.S. Administration's optimistic view, based in part on assessments by analyst Leon Gouré, that the war was winnable because of low enemy morale, Kellen joining with RAND colleagues in writing an open letter to the U.S. government recommending that troops be withdrawn.^[1] In a 1971 analysis, Kellen disagreed with many other observers and concluded that "nothing seems more unlikely" than that the communists would strike in a big way once the US forces were down to 200 thousand or less.^[8] Contrary to his forecast, the communists launched the bigger than ever Easter Offensive in South-Vietnam in March 1972. In 1972, months before the U.S. pullout pursuant to the Paris Peace Accords, investigative journalist Jack Anderson cited the open letter authored by Kellen as authority for the assertion that communist military morale in Vietnam was high.^[9]

Kellen went on to become an analyst of terrorism, his reports from the late 1970s and early 1980s identifying trends in terrorism that manifested a decade later.^[7]

Kellen wrote research papers and newspaper commentaries, as well as books including the biography Khrushchev (1961) and The Coming Age of Woman Power (1972), a study of male-female relationships.^[4] In 2003 he published his autobiography, Katzenellenbogen, named after his original family name. He lived in retirement in Los Angeles,^[1] dying at his home in Pacific Palisades.^[3]

Terrorism expert Brian Jenkins said Kellen often took a contrarian or independent view among RAND Corporation colleagues.^[4] In 2013, six years after Kellen's death, journalist and author Malcolm Gladwell noted Kellen's life and career were based on his ability to listen, citing him as "a truly great listener" for his ability to listen without filtering what he heard through the biases of the times.^[1]

• List of reports prepared for RAND Corporation