Grenstad was commissioned as a naval officer in 1980 after graduating from the
Royal Swedish Naval Academy at
Näsby Castle. Before that, he had completed a
circumnavigation trip with during 1977/1978. During his career as a naval officer he has held positions as corvette commander and commanding officer of the
3rd Surface Warfare Flotilla in
Karlskrona. Grenstad did his staff course at the
Military Academy Karlberg between 1994 and 1996. On 1 July 2005, Grenstad was appointed
Inspector of the Navy He left the post in 2013 On 25 November 2013, Grenstad was assigned to the
Chief of Policy and Plans Department at the
Swedish Armed Forces Headquarters. On 1 March 2014, Grenstad assumed the position of
Deputy Chief of Joint Operations. During this time, in October 2014, the Swedish Navy carried out an in response to a
foreign submarine incursion into Swedish territorial waters. On 1 February 2016, became Acting
Chief of Joint Operations and he served in this position until 1 June 2016 when he assumed the position of director of the Swedish Armed Forces Veterans' Unit. He there in this position until he again was appointed Commanding Officer of the Swedish delegation to the Neutral Nations Supervisory Commission (NNSC) on 1 March 2017. ==Personal life==