He was then employed as a labourer in the Cape Town Municipality from where he was advanced to administrative assistant and later elected to the Executive Committee of the Cape Town Workers Association. He became General Secretary of the Cape Town Municipal Workers Union. He was, subsequently, involved in the formation of the SA Municipal Workers Union and was elected unanimously as its first General Secretary moving from a local to a national position.
Mr Ernstzen participated in labour unity talks that led to the establishment of the Congress of South African Trade Unions (COSATU) hence his election to its first National Executive Council in 1985. He stayed on until becoming advisor to the Minister for the Public Service and Administration in 1994 and later Commissioner of the PSC. He was appointed by the State President to the Judicial Services Commission, which recommends appointment of judges to the Constitutional Court, High Court of Appeal, Labour and Labour Appeal Court and High Courts amongst other functions connected with the judiciary. Mr Ernstzen has just completed a second five year term of Office and continues to serve on the Commission.
Other positions he held included: Vice-Chair cum Chairperson of the Industrial Council for Local Government, Western Cape Province; elected first Chairperson of National Local Government Labour Relations Forum, elected to the African Regional Advisory Committee and to the International Executive Committee of Public Services International with membership in 36 countries world wide and was the first South African to be elected to these prestigious positions.
In January 2004, Mr Ernstzen was re-appointed Deputy Chairperson of the PSC.