Fest was born in Berlin, the son of Johannes Fest, a conservative Roman Catholic and strongly anti-Nazi schoolteacher who was dismissed from his position when the Nazis came to power in 1933. In 1936, when Fest turned ten, his family refused to make him join the Hitler Youth, a step which could have had serious consequences, although membership did not become compulsory until 1939. As it was Fest was expelled from his school, and then went to a Catholic boarding school in Freiburg im Breisgau in Baden: here he was able to avoid Hitler Youth service until he was 18.
The fact that his father, an "ordinary German," had understood the nature of the Nazi regime, and had resisted it, colored Fest's view of his fellow Germans for the rest of his life. He never accepted that Germans had not known what Hitler was doing or that they could not have resisted the Nazi regime.
In December 1944, when he turned 18, Fest decided to join the German Army, mainly to avoid being conscripted into the Waffen SS. His father opposed even this concession, saying that "one does not volunteer for Hitler's criminal war." His military service in World War II was brief and ended when he was made a prisoner of war in France. After the war he studied law, history, sociology, German literature, and art history at Freiburg, Frankfurt am Main and Berlin.
On graduating he started work for the American-run Berlin radio station RIAS (Radio In the American Sector), where from 1954 to 1961 he was editor in charge of contemporary history. During this time he was asked to present radio portraits of the main historical personalities influencing Germany from Bismarck to World War II, including such senior figures of the Nazi regime as Heinrich Himmler and Josef Goebbels. These portraits were later published as his first book The Face of the Third Reich: Portraits of the Nazi Leadership. In 1961 Fest was appointed editor-in-chief of television for the North German broadcasting service Norddeutscher Rundfunk (NDR), where he was also responsible for the political magazine Panorama. He resigned after a disagreement with left-wingers who eventually came to dominate the magazine.
Fest then embarked on his most important work, his biography of Adolf Hitler, which was published in 1973: It was the first major Hitler biography since that of Alan Bullock in 1952 and the first by a German writer. It came at a time when the younger generation of Germans was confronting the legacy of the Nazi period, and was both very successful in commercial terms and immensely influential. It sparked controversy among German historians, because Fest, a political conservative, rejected the then-dominant left-wing view that the causes of Hitler's rise to power had been largely economic.
Fest served as the editorial aide for Albert Speer, Hitler's court architect and later Minister for Munitions, when Speer was working on his autobiography, Inside the Third Reich (1970). After Speer's death, amid controversy over the reliability of the memoirs, Fest wrote Speer: The Final Verdict (2002), in which he criticised Speer for his knowing complicity in the crimes of the Nazi regime, something he successfully concealed at the time of the Nuremberg Trials. This echoed the verdict of Gitta Sereny in her major work Albert Speer: His Battle With Truth. (1995)
Fest's other major work of German history was Plotting Hitler's Death: The German Resistance to Hitler (1994), written to mark the 50th anniversary of the July 20 plot to kill Hitler. This work marked a partial reconsideration of his earlier harsh verdict on the German people. He acknowledged that many Germans had opposed the Nazi regime within the limits imposed on them by their circumstances. He maintained his view, however, that the majority of Germans had wilfully refused to accept the truth about Nazism until it was too late.
In 2002 he published Inside Hitler's Bunker: The Last Days of the Third Reich, a work based on newly-available evidence following the opening of the Soviet archives, but which largely confirmed the account of Hitler's death given in Hugh Trevor-Roper's book The Last Days of Hitler (1947). Inside Hitler's Bunker, along with the memoirs of Hitler's personal secretary Traudl Junge, formed the source material for the 2004 German film Der Untergang (Downfall), the third postwar German feature film to depict Hitler directly.
After the success of the Hitler biography Fest was invited to become co-editor of the Frankfurter Allgemeine Zeitung, a leading German newspaper based in Frankurt and one of the most potent political and cultural institutions in the German-speaking world. From 1973 to 1993 he edited the culture section of the paper. His views were generally conservative, pessimistic and sceptical, and he was particularly critical of the left-wing views that dominated German intellectual life from the late 1960s until the collapse of communism in 1991. He took a leading role in the "historians' dispute", in which he was identified with those rejecting what they saw as the Marxist hegemony in German historiography in this period.
Shortly before his death, Fest became embroiled in a public dispute with the left-wing writer and Nobel Prize winner Günter Grass, who had admitted in his autobiography that he had joined the Waffen SS in the last months of World War II. Fest criticised Grass not so much for having joined, but for having concealed the fact for so many years while engaging in political criticism of others over their Nazi pasts. He said: "After 60 years, this confession comes a bit too late. I can't understand how someone who for decades set himself up as a moral authority, a rather smug one, could pull this off.
Joachim Fest was married and had two sons and a daughter; all his children followed him into publishing or the media. He died at his home in Kronberg im Taunus near Frankfurt.