After active service in the Greco-Turkish war (1897), Metaxas completed his military training in Germany. He distinguished himself on the Greek general staff during the Balkan Wars (1912–13) and was appointed chief of staff in 1913 and promoted to general in 1916. During World War I he unsuccessfully fought to maintain Greek neutrality and opposed the plans of premier Eleuthérios Venizélos (Venizélos, Eleuthérios) for the conquest of western Anatolia, accurately predicting the military catastrophe that ultimately overtook the Greek offensive in Anatolia in 1921–22. Strongly monarchist in his politics, he left the country after the deposition of King Constantine (Constantine I) (1917), but returned after the King's restoration in 1920. With the fall of the monarchy in 1923, Metaxas again temporarily left Greece, but later held ministerial office under the republic (1928).

During the following years Metaxas provided doughty opposition to the government at the head of a small ultraroyalist party; shortly after the monarchy's restoration in 1935 King George II appointed him premier (April 13, 1936). Finally, on Aug. 4, 1936, he inaugurated a dictatorship under royal authorization. His Fourth of August Regime vigorously suppressed political opposition and succeeded in carrying out some beneficial economic and social reforms. When Italy invaded Greece in 1940, Metaxas brought a united country into the Western alliance. He remained in power until his death.