He was born in a working-class family of watchmakers with socialist beliefs. His grandfather was a member of the International Workingsmen's Association. Humbert joined the Social Democratic Party of Switzerland (Swiss Socialist Party) in 1911, at the age of 20. After studying Protestant theology in Neuchâtel, Paris and Berlin, he wrote a thesis about Socialism and christianism. He became a pastor in 1914 and started writing in the socialist daily newspaper La Sentinelle soon after.^[2][3]

He married Eugénie (Jenny) Perret in 1916, who would accompany him throughout his political life; she became known as Jenny Humbert-Droz.^[4]

Humbert-Droz opposed the First World War and refused to serve in the Swiss Army, for which he was imprisoned.^[5] He received another jail sentence for his participation the 1918 Swiss general strike.^[3] He supported the Bolshevik Revolution and travelled with Walther Bringolf to Russia to represent the left wing of the Social Democratic Party of Switzerland.^[6] There, both of them joined the Provisional International Bureau of the Kultintern.

In 1921, at the Third International Congress of the Comintern, Humbert-Droz was elected secretary of the Communist International, on the proposal of Vladimir Lenin himself.^[3] Humbert-Droz became after 1920 an outstanding leader in the international communist movement and travelled all around the world to organize the national sections of the Comitern. He exerted some control over the French Communist Party and called himself the eye of Moscow in Paris. He eventually became the first director of the Latin Secretariat of the Comintern.^[7] He was an ally and friend of Nikolai Bukharin. After the Sixth Comintern Congress, Bukharin was politically isolated and only few people in the Moscow Apparatus stayed loyal to him, including Humbert-Droz,^[8] who was disgraced along with his ally.^[7]

Their friendship later ended. He stated in his 1971 Mémoires that it was because in his last encounter with Bukharin in 1929, Bukharin said that he had sought to establish contact with Zinoviev and Kamenev in order to enlist their support in removing Stalin from the leadership and that Bukharin also told him he was planning to use individual terror (assassination) against Stalin. Humbert-Droz writes that he replied to Bukharin by criticising any rapprochement with Zinoviev and Kamenev and argued that resorting to individual terror would destroy the Bolshevik leadership.^[9]

Boukharine me dit aussi qu'ils avaient decide d'utiliser la terreur individuelle pour se debarrasser de Staline.^[10] [Bukharin also told me that they had decided to use individual terror in order to rid themselves of Stalin].

He managed to re-enter the Executive Committee of the Communist International after self-criticizing and capitulating. In 1943, he was definitely expelled from the Swiss Communist Party.^[5] Aware of his worth and experience, the Swiss Socialist Party invited him to return to his original party, where he was party secretary until 1959 and then secretary of the Neuchâtel cantonal section of the party until 1965.

He retired to La Chaux-de-Fonds but remained politically active. He fought the atomic armament of Switzerland and contributed to various journals. Toward the end of his life, he undertook the writing of his memoirs, published between 1969 and 1973.^[2]

• L’œil de Moscou à Paris, 1922-1924 (1964)
• L'origine de l'Internationale communiste : de Zimmerwald à Moscou (1968)
• Mémoires

1, Mon évolution du tolstoïsme au communisme, 1891-1921 (1969)

2, De Lénine à Staline, dix ans au service de l'Internationale communiste, 1921-1931 (1971)

3, Dix ans de lutte antifasciste : 1931-1941 (1972)

4, Le couronnement d'une vie de combat : 1941-1971 (1973)