He became a member of the first class to attend the Military Academy and later graduated from the Military Staff College.
In 1902 he joined the General Staff Office and was assigned to duty in Germany to pursue military studies. 'With the outbreak of the Russo-Japanese War he returned to Japan and served as a staff officer at the front. After the war, he was once more assigned to duty in Germany ( 1906—08 ) and in 1910 advanced to the rank of colonel. In 1911 he became a section chief in the Bureau of Military Affairs in the War Ministry and, under the direction of Chief of the Bureau of Military Affairs Tanaka Giichi (later war minister and prime minister), was active in setting up two new army divisions, attracting attention as a figure of importance in military circles.
In 1913, when the Yamamoto Gombei cabinet abolished the practice of filling the posts of minister of war and navy with officers on active duty, Ugaki opposed the move and distributed an anonymous pamphlet expressing his views. As a result he was for a time demoted to the post of regimental commander in Nagoya, but in 1915 was reassigned to duty as a section chief in the Bureau of Military Affairs. He advanced to the rank of major general, and in 1916', on the recommendation of Vice-Chief of the General Staff Tanaka, he became chief of the strategy division of the General Staff Office. In 1919 he became head of the Military Staff College and a lieutenant general and in 1921 became division commander in Himeji. The following year he became head of the main office of the Department of Military Education, in 1923 he became vice-minister of war (under Tanaka as minister of war), and in 1924 was appointed minister of war in the cabinet of Kiyoura Keigo, a post that he continued to hold in the Kato Takaaki and Wakatsuki Reijiro cabinets as well.
In 1929 he became minister of war in the Hamaguchi Osachi cabinet and gradually organized his own clique within the army, and in 1931 a coup d’etat was actually planned for the purpose of putting Ugaki in power, though it was called off before the date for execution (the so-called March incident). The same year, he was appointed governor-general of Chosen (Korea) and entered the reserve. He resigned the post in 1936 and in 1937 was ordered to form his own cabinet, but he met with violent opposition from the principal leaders of the army, and the plan came to nothing.
Following the outbreak of hostilities between China and Japan, he was appointed foreign minister after the reorganization of the Konoe Fumimaro cabinet in 1938, as well as minister of overseas affairs, and attempted to restore peace with China, but his efforts proved unsuccessful. Because of his opposition to the establishment of the Koain, he resigned after little more than four months in office. Thereafter, he retired from political life. At the conclusion of the Pacific War, he was barred from public office. In 1953, when the ban was ended, he ran for a seat in the Upper House of the Diet and received the largest number of votes of any candidate in the country. He was eighty-five at the time, but his health was failing, and he died three years later.