Born in Saga Prefecture in 1876, Masaki graduated from the 9th class of the Imperial Japanese Army Academy in 1897 and was commissioned as a second lieutenant in the IJA 46th Infantry Regiment in June 1898.

During this period, he was promoted to captain in June 1904; however, he found the war a highly traumatizing experience, and after his return to Japan, Masaki wrote that he had considered resigning from the Army and entering the Buddhist priesthood.

With Heisuke Yanagawa and Hideyoshi Obata, the group merged with the rival Tōseiha faction under Kazushige Ugaki to dominate the Japanese army throughout the 1930s until World War II.

A supporter of an alliance with Nazi Germany and Fascist Italy, Masaki continued his involvement with the Imperial Way Faction until his forced retirement though political maneuvers by General Tetsuzan Nagata.

Dissatisfaction with Masaki's forced retirement resulted in the assassination of Nagata the following year which, in turn, led to the February 26 Incident of 1936.