Yokoyama was born in Fukuoka prefecture as the eldest son of a village mayor. He graduated from the 24th class of the Imperial Japanese Army Academy in 1912 and the 37th class of the Army Staff College in 1925. His rise through the ranks was steady and rapid, serving in mostly staff and administrative posts within the Chosen Army and Imperial Japanese Army General Staff. aside from a brief combat deployment during the Japanese intervention in Siberia and an observation tour in Europe in 1934. He was as appointed commander of the Kwantung Army's Railway Zone serving in that post until 1937. With the start of the Second Sino-Japanese War, Yokoyama was appointed commander of the IJA 2nd Infantry Regiment from March 1938 to March 1939, whereupon he was promoted to major general. He was assigned command of the 2ndField Railway, followed by the 1st Field Railway until his promotion to lieutenant general in 1941. In June 1942, Yokoyama was assigned command of the Manchukuo-based IJA 8th Division, which was garrisoned in eastern Heilongjiang guarding the border with the Soviet Union. However, in 1944, as the situation in the Pacific war continued to deteriorate for the Japanese, portions of the IJA 8th Division were sent to Truk, where they were largely annihilated by lack of food and American air raids. The remainder of the division was transferred from Manchukuo to the Philippines under the command of General Yamashita Tomoyuki's 14th Area Army, and under the name of “Shimbu Group”, was wholly made responsible for the defense of southern Luzon. Suffering severe casualties, the remnants of the command was assigned to the IJA 41st Army in March 1945. Yokoyama was responsible for defending Manila against the U.S. Sixth and Eighth Armies from 3 February until his surrender on 4 March 1945, by which time his army had been reduced to just 6500 men. At the end of the war, Yokoyama was arrested, taken before a military tribunal in Manila, and was charged with war crimes due to the various atrocities committed by Japanese forces during the Japanese defense of Manila and was sentenced to death. He was pardoned by Philippines PresidentElpidio Quirino and allowed to return to Japan in 1953. He died in 1961.
