Still a relatively youthful man upon his defeat in 1932, the fifty-eight-year-old former President lived another thirty-two years before his death on October 20, 1964. Immediately after the inauguration of Franklin Roosevelt, Herbert Hoover retreated to his home in Palo Alto, California. For much of the 1930s—and, indeed, for decades to come—the public, and especially the Democratic Party, blamed Hoover for the Great Depression. Likewise, few Republicans in the 1930s wanted Hoover involved in party politics because of his negative standing in the popular mind. Wealthy and generous, Hoover did not need to work, but even the fishing that he loved could consume only so many hours of the week. From his home in Palo Alto, Hoover launched a series of bitter attacks on the New Deal in letters and essays, condemning many of its programs as "fascistic." Roosevelt's decisions to abandon the gold standard, to recognize the Soviet Union, to pack the Supreme Court, to support federal government intervention in the economy, and to build the foundations of a welfare state angered and worried Hoover. In the 1936 presidential campaign, he actively supported Kansas Republican candidate Alfred M. Landon, who lost to Roosevelt by a wide margin.

While on a tour of Europe in 1938—Hoover traveled extensively in his post-presidential years—Hoover met with with Adolf Hitler. The former U.S. President dressed down the German dictator, irritated at Hitler's shouting in their private audience. Still, Hoover, recalling what he regarded as the needless bloodshed of World War I, opposed U.S. entry into the European conflict that broke out in 1939 after Germany attacked Poland. Japan's surprise attack on Pearl Harbor, however, changed his mind. As in 1918, war created a need for Hoover's organizational and humanitarian skills. FDR put aside his personal antipathy towards his predecessor and supported Hoover's appointment to chair an international relief organization for Poland, Finland, and Belgium. Hoover, however, was unsuccessful in getting food relief to nations occupied by the Nazis.

In the post-World War II years, Hoover remained committed to public service and to commenting on both domestic and international affairs. For the Truman administration, Hoover served as coordinator of the Food Supply for World Famine in 1946 and advised the U.S. government on occupation policies in Germany and Austria. In 1947, a Republican-led Congress named Hoover chairman of the Commission on the Organization of the Executive Branch of Government, which became known as the "Hoover Commission." The Hoover Commission, which congressional Republicans hoped would curtail (if not destroy) FDR's New Deal policies and programs, instead recognized the greater burdens of the modern presidency and proposed bureaucratic and administrative reforms to strengthen the Executive Branch. Indeed, a number of the Commission's proposals found their way into the 1949 Executive Reorganization Act. In 1955, Hoover reprised his role as head of a commission to study government organization, although this later commission is generally considered less successful than the first. Nonetheless, Hoover made his own, unique contribution to the construction of the modern presidency.

It was in international affairs in the 1940s and 1950s, however, that Hoover attracted the most attention. He opposed the use of the atomic bomb on Japan, of which he wrote to a friend in August 1945: "The only difference between this and the use of poison gas is the fear of retaliation. We alone have the bomb." Hoover's commentary on America's Cold War policies was at times supportive and at other times highly critical. For instance, he encouraged Truman's policies in the immediate aftermath of World War II to rebuild Germany, both economically and politically, as a barrier against Soviet communism.

But Hoover grew disaffected with the administration's Cold War policies which demanded ever greater military commitments in both Asia and Europe. He criticized Truman's decisions to intervene in the Korean War and to station four additional American divisions in Europe after the outbreak of hostilities on the Korean peninsula in 1950. Hoover became a leading voice among a group of influential Republican conservatives, including Senate majority leader Robert Taft of Ohio, who advocated building American naval and air power and basing American military strategy on a defense of the Western Hemisphere. Hoover supported such a position because he feared that massive land wars in Europe and Asia would cost millions of American lives, because he believed that Europeans could do more to defend themselves against the Soviets, and because he thought that the cost of maintaining the global commitments outlined by the Truman administration would bankrupt the United States. Hoover and Taft, of course, ended up on the losing side of what came to be known as "The Great Debate" concerning America's Cold War foreign policy.

Hoover remained active in party politics as well. He supported the candidacies of Robert Taft in 1948 and 1952, and of Dwight D. Eisenhower in 1956. He was less enthusiastic about Vice President Richard Nixon's run for the presidency in 1960. In 1964, shortly before his death, Hoover endorsed Senator Barry Goldwater for President, telling his associates that the conservative Arizona Republican closely mirrored his own views on the need for limiting federal authority over everyday life and the American economy.

Herbert Hoover died in 1964, at the age of ninety, from colon cancer. He was laid to rest in West Branch, Iowa, beside his wife Lou, who passed away in 1944.