After retirement, Yonashiro became a manager for Yomiuri in 1986. He led the club to won 1986/87 Japan Soccer League. The club also won 1986 and 1987 Emperor's Cup. In Asia, the club won 1987 Asian Club Championship. He resigned in 1990. In September 1994, he became a manager for Kyoto Purple Sanga as Seishiro Shimatani successor. In June 1996, he became a manager for Kyoto Purple Sanga again as Oscar Bernardi successor. In 2004, he signed with Prefectural Leagues club FC Ryukyu. He promoted the club to Regional Leagues in 2005 and Japan Football League (JFL) in 2006. In 2007, he moved to Regional Leagues club Giravanz Kitakyushu. He promoted the club to JFL in 2008 and J2 League in 2010. He resigned end of 2010 season. In 2013, he signed with JFL club Blaublitz Akita. He promoted the club to new league J3 League in 2014. He resigned end of 2014 season.