After graduating from Harvard Law School, Komura returned to Japan and entered the Japanese Ministry of Justice (1880), later transferring to the Ministry of Foreign Affairs. A year before the Sino-Japanese War (1893), he became a chargé d'affaires in Beijing. Subsequently, Komura served in Korea, the United States, Russia, and again in China.

In 1901–05 Komura was minister of foreign affairs and tirelessly negotiated for the Anglo-Japanese Alliance (1905), which became a major basis of Japanese diplomacy in the ensuing years. As special envoy, Komura concluded the Treaty of Portsmouth (Portsmouth, Treaty of) (1905), which settled the Russo-Japanese War. Foreign minister again (1908) in the second Katsura Cabinet, he pursued treaty negotiations with Western nations and saw completion of the annexation of Korea. In 1910 he was created marquess.