In 1976, when Yokoyama played for Mitsubishi Motors (later "Urawa Reds"), he became a playing manager as Hiroshi Ninomiya successor. In 1978, the club won all three major titles in Japan; Japan Soccer League, JSL Cup and Emperor's Cup. It was first domestic treble for a Japanese club. The club also won 1980 Emperor's Cup, 1981 JSL Cup and 1982 Japan Soccer League. He resigned in 1984. In 1988, he became a manager for the Japan national team as Yoshinobu Ishii successor, where he coached Japan in the country's first competitive tournament, the 1988 AFC Asian Cup. At 1990 World Cup qualification in 1989, Japan lost in First round. Although Yokoyama managed at 1990 Asian Games, he resigned in 1991. In 1994, he became a manager for Urawa Reds as Takaji Mori successor. However, the club finished at the bottom in 1994 season and he resigned end of season. In 1995, he became a general manager. From October 2000, he managed the club. In 2002, he resigned as general manager.