Namir became the secretary of the Ahdut HaAvoda party in 1926, a position he held until 1930. From 1929 to 1935 he also served as the director of the statistics department in the Histadrut. From 1935 he was a member of Tel Aviv's city council and from 1936, secretary of the workers' union in the city. Additionally, he was a member of the Haganah command in the city and later in all of Mandatory Palestine.
Between 1949 and 1950 he was also on a diplomatic mission to Eastern Europe and Moscow, about which he wrote a book in 1971 (A Mission in Moscow: A Honeymoon and Years of Wrath).
From 1950 to 1956, Namir served as the general secretary of the Histadrut. He was elected to the Knesset on behalf of Mapai in 1951 and remained an MK until 1969, when the 6th Knesset was disbanded. During this time he was also the CEO of the government-owned construction company Amidar (1959-59) and Minister of Labour from 1956 until 1959.
In addition to his Knesset duties, Namir was also the mayor of Tel Aviv between 1960 and 1969, and part of a major expressway in the city (Highway 2, also called Derekh Namir) and a square bear his name.
He was married to former Knesset Member and Minister Ora Namir.