John Ernstzen was the first general secretary of SAMWU. He played a key role in building up SAMWU. At the age of 22 in 1962 John was employed by the Cape Town City Council as a laborer in the Parks and Forest Department. In the same year, he joined the Cape Town Municipal Workers Association (CTMWA).John together with other comrades transformed the CTMWA into active democratic and non-racial union.
In 1968 he was elected as the full-time General Secretary of CTMWA. Under his leadership the CTMWA joined UDF and became active in the unity talk that eventually led to the formation of COSATU in 1985. John served in the first COSATU CEC and NEC in 1985.When SAMWU was formed on COSATU instructions in 1987, the CTMWA become a key component of the new union and John was elected as the first General Secretary of SAMWU. After 1994, John became a special Advisor to the Ministry for Public Service and Administration and served on various boards.