From 1932–38, he was Secretary of the Tel Aviv Workers' Council. In the summer of 1935, he served for a few months as the envoy for the Halutz organization in Nazi Germany until he was expelled by the Gestapo. From 1938–39, he was Secretary of Mapai.
In 1940, he enlisted in the British army to fight against Nazi Germany in World War II, where he reached the rank of Major. He was captured in the Greek front in 1941, along with other soldiers from the Yishuv, until they were released in 1945.
After the war, he joined Mapam and from 1948 became one of its leaders. After the split in Mapam in 1954, he became one of the leaders of its left-splinter, Ahdut HaAvoda party.
From 1969–73, he became General-Secretary of the Histadrut. He was a Knesset member seven times, and member of several parliamentary committees. From 1958–62, he became the Minister of Transport, but resigned over what he called the government's anti-labour socioeconomic policy. In 1977, he retired from active political life, but continued to express critical, pro-socialist views for the rest of his life.
He was the author of several books and articles. On 19 May 2006, he died in his kibbutz. Ben-Aharon donated his body to science, so there was no burial.
Responding to Ben-Aharon's death, President of Israel, Moshe Katzav stated that: "Israel has lost one of its builders and shapers of its social character." Prime MinisterEhud Olmert said that "the State of Israel has lost one of its giants, a true zionist and honest ideologue, who during tens of years did not hesitate to express his unique and penetrating views." Israel's Vice Prime Minister, Minister for the Development of the Negev and Galilee, Shimon Peres said that: "One of the spiritual fathers of the Israeli labour movement has left us." The Minister of Defense, Amir Peretz, said: "today, one the giants of the State of Israel has left us. If there is a man that can be said to have been one of the titans of the generation, it is Yitzhak Ben-Aharon."