Having finished his elite secondary education in 1917, Heinemann briefly became a soldier in the First World War, but his severe illness stopped him from being sent to the front.

From 1918, he studied law, economics, and history at the universities of Münster, Marburg, Munich, Göttingen, and Berlin, graduating in 1922 and passing the bar in 1926. He received a Ph.D in 1922 and a doctorate of law in 1929.

He heard Hitler speak in Munich in 1920 and had to leave the room after interrupting Hitler’s diatribe against the Jews.

In 1926, Heinemann married Hilda Ordemann (1896-1979), who had been a student of Rudolf Bultmann, the famous Protestant theologian. His wife and the minister of his wife’s parish, Wilhelm Graeber, led Heinemann back to Christianity from which he had become estranged. Through his sister-in-law, he became acquainted with Swiss theologian Karl Barth, who strongly influenced him such as in his condemnation of nationalism and antisemitism.

At the beginning of his career, Heinemann joined a renowned firm of solicitors in Essen. In 1929, he published a book about legal questions in the medical profession. From 1929 to 1949, he worked as a legal adviser to the Rheinische Stahlwerke in Essen, and from 1936 to 1949, he was also one of its directors.

In 1930, Heinemann joined the Christlich-Sozialer Volksdienst (“Christian Social People’s Service”), but he voted for the Social Democratic Party in 1933 to try to prevent a victory of the NSDAP.

Heinemann was an elder (Presbyter) in Wilhelm Graeber’s parish in Essen, when Graeber was sacked in 1933 by the new church authorities who co-operated with the Nazis. Opposition against those German Christians came from the Confessing Church, and Heinemann became a member of its synod and its legal adviser. As he disagreed with some of the developments within the Confessing Church, he withdrew from the church leadership in 1939, but he continued as an elder in his parish, in whose capacity he gave legal advice to persecuted fellow Christians and helped Jews who had gone into hiding by providing them with food.

He was also invited to join the board of directors of the Rheinisch-Westfaelisches Kohlesyndikat in 1936, but he refused, as he was expected to end his work for the Confessing Church.

From 1936 to 1950, Heinemann was head of the YMCA in Essen.

In August 1945, he was elected as a member of the Council of the Evangelical Church in Germany. The Council issued the Stuttgart Declaration of Guilt in October 1945 in which it confessed guilt for the failure of the Protestant church not to oppose the Nazis and the Third Reich. Heinemann regarded the declaration as a “linchpin” in his work for the church.

After the Second World War, the British authorities appointed Heinemann mayor of Essen, and in 1946, he was elected to that office, which he kept until 1949. He was one of the founders of the Christian Democratic Union in North Rhine-Westphalia, in which he saw an interdenominational and democratic association of people opposed to Nazism. He was a member of the North Rhine-Westphalian parliament (Landtag, 1947–1950), and from 1947 to 1948, he was Minister of Justice in the North Rhine-Westphalian government of CDU Prime Minister Karl Arnold.

From 1949 to 1955, Heinemann was president of the all-German Synod of the Protestant Churches of Germany. He was among the founders of the German Protestant Church Congress (Deutscher Evangelischer Kirchentag), a congress of the Protestant laity. In 1949, he was also one of the founding editors of Die Stimme der Gemeinde (“The Voice of the Congregation”), a magazine which was published by the Bruderrat (Brethren’s Council) of the Confessing Church. In the World Council of Churches he belonged to its “Commission for International Affairs”.

When Konrad Adenauer became the first Chancellor of the newly founded Federal Republic of Germany in 1949, he wanted a representative of the Protestants in the CDU in his government. Heinemann, the president of the Synod of Protestant Churches, reluctantly agreed to become the Minister of the Interior although he had planned to resume his career in industry.

In October 1950 Heinemann had started practising as a lawyer again. In court, he predominantly represented political and religious minorities. He also worked for the release of prisoners in East Germany. Later, he defended conscientious objectors to compulsory military service and Jehovah’s Witnesses in court. The latter refused to do even community work instead of military service because of their absolute conscientious objection.

Heinemann left the CDU, and, in 1952, he founded his own political party, the All-German People’s Party (Gesamtdeutsche Volkspartei). Among its members were such politicians as future Federal President Johannes Rau and also Erhard Eppler. They advocated negotiations with the Soviet Union with the aim of a reunited, neutral Germany between the blocs, but the GVP failed to attract many voters. Heinemann dissolved his party in 1957 and joined the Social Democratic Party of Germany (SPD), whose aims were relatively close to his own.

In March 1969 Gustav Heinemann was elected President of West Germany. As he was elected with the help of most delegates of the Free Democratic Party (FDP/Liberals) his election was generally understood as a sign of the re-orientation of the FDP with regard to a future coalition with the SPD (Social-liberal coalition, October 1969 – October 1982).

It was Heinemann’s idea to found a museum for the commemoration of German liberation movements, and he was able to open such a place officially in Rastatt in 1974. His interest in that subject was partly from the involvement of his own ancestors in the revolution of 1848.

On account of his age and fragile health, he did not stand for a possible second term as President in 1974. He died in 1976.