During the early 1950s he witnessed the human rights abuses of Palestinians in southern Israel by the Israeli authorities. In 1956, following his participation as a mechanic with the Israeli forces in the Sinai Campaign, he broke from Zionism and became active with the Israeli Communist Party. As a result, he was forced to leave Urim in 1958 and he moved with his family to the southern Israeli town of Beersheba.
In 1963 Flexer moved to Canada settling in Winnipeg, Manitoba where he became involved in the anti-war movement protesting the Vietnam War. In 1968 he moved to Montreal. Following a brief stay in Israel in 1970, where he lived and worked on kibbutz Gan-Shmuel, he moved back to Canada and settled in Toronto in 1970. There he joined the Waffle, a radical socialist tendency within the New Democratic Party, becoming its provincial organizer in Ontario. Moving leftward, he helped form the Red Circle, a Marxist tendency within the Waffle. When the Waffle was forced out of the NDP in 1972, Flexer and the Red Circle split with the Waffle, opposing its decision to leave the NDP, and tried to continue Marxist activities within the NDP.
Flexer and the Red Circle joined the Revolutionary Marxist Group in 1973 which, in turn, joined with other Trotskyist groups to form the Revolutionary Workers League in 1977. In 1973, Flexer, a diesel mechanic and machinist by trade, was hired by Carruthers, the main Caterpillar service centre and dealership in Southern Ontario. There he joined the Canadian Auto Workers union local 112, and became a shop Steward and then plant chairman for the union.
Flexer led a small industrial caucus within the RWL. He joined the exodus of Trotskyists that left the RWL in the early 1980s and focused instead on working within the CAW. He freelanced in the CAW's education department where he helped develop the union's political education program for workers and taught Marxist Economics in the CAW's Port Elgin, Ontario education centre. When asked what political party he belonged to, he'd joke he was a member of the "Joe Flexer Communist Party, we have a very small membership but a very lively internal discussion".
He joined the Communist Party of Canada while it was in crisis due to the collapse of the Soviet Union and helped organize the split that saw Flexer and much of the CPC's leadership leave to form the Cecil-Ross Society.
Flexer opposed Bob Rae's NDP government following its introduction of the Social Contract that suspended collective bargaining contracts for public sector unions. In the 1995 Ontario election he ran as an "Independent Labour" candidate in Oakwood against NDP MPP Tony Rizzo and placed fourth out of seven candidates with 301 votes.
He subsequently joined Socialist Action, a Trotskyist group, and became a member of its editorial board. As Socialist Action practiced entryism, Flexer also joined the New Democratic Party which he had previously run against.
Flexer suffered from heart disease in the last years of his life. In 1994 he underwent a heart transplant. His new heart failed him after six years and he died in Toronto.
It would be a mistake of great proportion to look upon Flexer as a simple communist agitator. He was much more than that. He was a fierce class warrior who made his bosses tremble because he was not just armed with ideas, he was armed with the ability to communicate those ideas on the shop floor and by his actions, people would follow him through hell. He was truly a charismatic human being who relished the fight against capitalism until the second he died. His last words were an attack on the Zionists whom he despised with a special fervor. Dedicated to the ideas of socialism and the inherent humanist perspective that goes along with that, his politics were not that of a middle class intellectual playing at revolutionary games, Flexer was the real thing. He could be trusted to do what he said he would do.
Joe knew how to live life to the fullest. He became a great cook and loved to entertain. He went to folk festivals for fun because he loved music. He was a great storyteller in English and Yiddish and Arabic and Hebrew. He equally understood pathos and joy and his own life reached highs and lows with amazing rapidity. He was a loving father of four children who all were with him when he passed away. He was a man who had friends who were real friends. Flexer died in a state of love which had eluded him until the last five years of his life. He had found happiness.