Nikolaos Tsounis was born in Athens, and entered the Hellenic Navy Academy in 1979, graduating in June 1983 as a line ensign.

[1] In 1988–1991 he served as operations and weapons officer on the fast attack boat Simitzopoulos, being promoted to lieutenant on 2 July 1990.

[1] In 2001–2004 he served in the Hellenic Navy General Staff as section chief, as commander of the frigate Adrias in 2004–05, and then as ACOS N1 at STRIKFORNATO in 2005–2008.

Promoted to commodore on 1 April 2012, he served as chief of Fleet Headquarters staff in 2012–13, and as head of the Frigates Command in 2013–14.

[1] Promoted to rear admiral on 10 March 2014, he served as chief of Fleet Headquarters in 2014–15, was promoted to vice admiral on 13 October 2015, director of the National Defence Academy on 20 October 2015, before being chosen as Chief of the Hellenic Navy General Staff on 16 January 2017.