Prior to coaching he was a lecturer of economics, political science and informatics at vocational colleges .

In 1996, he attained his UEFA Pro License (Fußball-Lehrer) from the Deutsche Sporthochschule in Köln, Germany.

[12] Middendorp then returned to Ghana to become head coach of Hearts of Oak between March 2004 and June 2004.

[1] Middendorp returned to Africa to become head coach the South African Castle Premiership team Kaizer Chiefs from the beginning of the 2005–06 season up to 4 March 2007.

[15] On 14 March 2007, Middendorp started his third stint as a coach at Arminia Bielefeld,[3] but was fired by club management on 10 December 2007.

[19] On 7 May 2009, Middendorp joined Cypriot club, Anorthosis Famagusta,[20] but was fired on 24 July 2009[21] following their elimination in the second round of the 2009–10 UEFA Europa League.

[34] He coached the Eastern Province SA Premier League team Chippa United from 5 January to 30 March 2015.

[36] In the 2015–16 season Ernst led not one but two South African Premier League teams from the relegation position to safety.

On 28 December 2015, Ernst returned to Maritzburg United[24] (a club he coached twice before)[24] who were then in position 16[38] and facing the stark danger of relegating.

Ernst and his team narrowed the gap until the final nail-biting game of the season when Maritzburg United moved to the safety of position 14.

[41] Kaizer Chiefs finished the following season in 2nd place and qualified for the 2020–21 CAF Champions League.

[46] On 16 August 2023, he resigned from this position after being relegated to the Regionalliga Nord and suffering two defeats in the first three games of the 2023–24 season.

[47] In September 2023, Middendorp was appointed as the head coach of Tanzanian Singida Fountain Gate replacing Hans van der Pluijm.

[48] He resigned after just two weeks and one game, a 1–0 win over Future in the 2023–24 CAF Confederation Cup qualifying rounds, due to disputes with the club management.

[49] He was appointed manager of South African premiership club Cape Town Spurs in November 2023.