He was born in Arolsen at the ruling family's castle, the eldest son and heir of Prince Friedrich of Waldeck and Pyrmont and his consort Princess Bathildis of Schaumburg-Lippe. He was the nephew of William II, King of Württemberg, and Emma of Waldeck and Pyrmont, Queen Regent of the Netherlands. He was also a cousin of Wilhelmina, Queen of the Netherlands, and Charles Edward, Duke of Saxe-Coburg and Gotha. He enlisted in the German Army as a cadet and fought in the First World War, where he suffered serious injuries. At the end of the war, his family lost their Principality as Waldeck became a Free State in the new Weimar Republic.
After the war, Josias studied agriculture. On 1 November 1929, he joined Adolf Hitler's Nazi Party, becoming a member of the SS on 2 March 1930. He was immediately appointed adjutant to Sepp Dietrich (a leading member of the SS), before becoming Heinrich Himmler's Adjutant and staff chief in September 1930. Josias was elected as the Reichstag member for Düsseldorf-West in 1933 and was promoted to the rank of SS Lieutenant General. He was promoted again in 1939, to the Higher SS and Police Leader for Weimar. In this position he had supervisory authority over Buchenwald concentration camp.
Buchenwald had first caught the attention of Waldeck in 1941. Glancing over the camp's death list, he had stumbled across the name of Dr. Walter Krämer, a head hospital orderly at Buchenwald. He recognized it because Dr. Krämer had successfully treated him in the past. The Prince investigated the case and discovered Karl Otto Koch, the Camp Commandant, had ordered both Krämer and Karl Peixof (a hospital attendant) killed as "political prisoners" because they had treated him for syphilis, a fact Koch wished to keep secret. Waldeck also received reports that a certain prisoner had been shot while attempting to escape. By that time, Koch had been transferred to the Majdanek concentration camp in Poland, but his wife, Ilse, was still living at the Commandant's house in Buchenwald. Waldeck ordered a fullscale investigation of the camp by Dr. Georg Konrad Morgen, an SS officer who was a judge in a German court.
Throughout the investigation, more of Koch's orders to kill prisoners at the camp were revealed, as well as evidence of embezzlement of property stolen from prisoners. It was also discovered that the prisoner who was "shot while trying to escape" had been told to get water from a well some distance from the camp, then was shot from behind; he had also helped treat Koch for syphilis. A charge of incitement to murder was lodged against Koch by Prince Waldeck and Dr. Morgen, to which later was added a charge of embezzlement. Other camp officials were also charged, including Ilse Koch. The trial resulted in a death sentence for the Commandant. He was executed by firing squad on 5 April 1945. Morgen was convinced that Ilse Koch was guilty of sadistic crimes, but the charges against her could not be proven. She was detained by German authorities until early 1945.
Adolf Hitler appointed Waldeck a member of the Ordnungspolizei in April 1941 and, a year later, he was appointed High Commissioner of Police in German-occupied France. One of his first acts in his new role was ordering French hostages to be placed on German troop trains, to discourage sabotage attempts on them. He was made a General in the Waffen-SS in July 1944. Waldeck was arrested on 13 April 1945, and sentenced to life imprisonment by an American court at Dachau on 14 August 1947. The first of the two successful charges against him alleged that he was personallly responsible for crimes at Buchenwald, since the camp was located in his jurisdictional area, notwithstanding the fact that he was never in command of it. The second charge was that he had ordered the execution of the Camp Commandant of Buchenwald, Standartenführer Karl Otto Koch, after it was discovered Koch had disgraced both himself and the SS.
The Buchenwald Camp Trial was also controversial. Although commonly referred to as "trials," these proceedings were technically not trials because the normal rules of court trials in America or Great Britain were not followed. Hearsay testimony was allowed and most of the prosecution witnesses were paid. Affidavits from witnesses were allowed, which meant that the defense had no opportunity to cross-examine the witness who had signed the affidavit. The accused were charged with participating in a "common plan" to commit war crimes and they were presumed to be guilty until proven innocent. They were not called "defendants" because the burden of proof was on them, not on the prosecution (as is customary in a court trial). Many of the accused claimed they had been beaten during interrogation.
Military Governor of Germany, General Lucius D. Clay ordered that the sentences of the Buchenwald Trial be re-examined on the basis of extensive records and, on 8 June 1948, confirmed fifteen of the death sentences and commuted seven. Most of the imprisonment sentences were also commuted, including Waldeck's (from life to twenty years). Waldeck was taken to Landsberg am Lech, where he served only three years of his sentence before being released in December 1950 for health reasons. He was granted an amnesty by the Minister President of Hesse in July 1953, which resulted in a significant reduction of the fine imposed on him. Josias became head of the House of Waldeck and Pyrmont upon the death of his father, on 26 May 1946, while under arrest. He died at his estate, Schloss Schaumburg, in 1967, and was succeeded as head of the house by his only son Prince Wittekind. His Serene Highness The Hereditary Prince of Waldeck and Pyrmont, (1896–1946). His Serene Highness The Prince of Waldeck and Pyrmont, (1946–1967). Obergruppenführer (as Gruppenführer) Josias. Prince Josias married Duchess Altburg of Oldenburg (1903–2001), a daughter of the former Grand Duke of Oldenburg, Friedrich August II, on 25 August 1922 at Rastede. They had five children :
- Princess Margarethe (b. 22 May 1923; d. 21 August 2003) married (1952) div. (1979) Count Franz August zu Erbach-Erbach (b. 1925)
- Princess Alexandra (b. 25 September 1924; d. 4 September 2009) married (1949) Prince Botho of Bentheim und Steinfurt (1924–2001)
- Princess Ingrid (b. 2 September 1931)
- Prince Wittekind (b. 9 March 1936) married (1988) Countess Cecilie of Goëss-Saurau (b. 1956)
- Princess Guda (b. 22 August 1939) married (1958) div. (1972) Frederick William, Prince of Wied (1931–2001); married second (1968) Horst Dierkes (b. 1939)