Middendorp joined the Dutch Army in 1979, attending officers training at the Koninklijke Militaire Academie in Breda.

Following this training he was posted to the Army staff and put in charge of developing and implementing new real estate management policy in light of the deactivation of the Dutch draft.

Completing this posting Middendorp attended the United States Army Command and General Staff College starting in 1996.

Following this command, Middendorp was promoted to colonel and posted to the Ministry of Defence as policy coordinator for the chief director of general affairs.

He was posted back to Afghanistan in February 2009 as commanding officer of the multinational Task Force Uruzgan (TFU6, specifically).

In preparation for his new role, Middendorp was promoted lieutenant general on 1 January 2012 and placed in charge of projects and operations at the Defence Staff.