After attending military preparatory schools in Kumamoto and Tokyo, Sonobe graduated from the 16th class of the Imperial Japanese Army Academy in 1904.

From January 1925 to July 1927, Sonobe was the Army representative with the Japanese diplomatic mission to the League of Nations.

In February 1938, the division was permanently assigned to Kwantung Army, but arrived too late to participate in the Battle of Lake Khasan in July 1938.

In August 1939, Sonobe was reassigned back to Japan and assigned to command the Central District Army, a reserve force with responsibility for defense of the Japanese home islands.

Sonobe was given another chance during the Second Sino-Japanese War, when we was assigned command of the Japanese Eleventh Army from March 1940.