Koyama, born in Sagamihara, Kanagawa as the youngest of two children, is the oldest member of NEWS and is often seen as the mother figure of the group. Before he was in NEWS, he was a member of BAD and J-Support, which was later renamed K.K.Kity, along with fellow NEWS members Hironori Kusano and Shigeaki Kato. As the leader of K.K.Kity, he was often an announcer and this has carried through to NEWS. Despite this, he actually only joined Johnny's Entertainment at the beginning of 2001, when he was 17 years old.
Upon entering Johnny's Entertainment on January 21, 2001, Koyama started out with the unit B.A.D. which stands for Beautiful American Dream. His second unit as a junior was B.B.D. which at first stood for Bad Boys Dancing, but was later changed to Best Beat Dancing. In October 2001, he was chosen as a member of Domoto Koichi's project, J Support, alongside fellow NEWS members Hironori Kusano and Shigeaki Kato. It was in this unit that Koyama started out as an announcer for shows such as the Ya-Ya-yah Show, and this has carried through to NEWS, which he joined in September 2003.
In 2007, he graduated from Meiji University's Department of History and Geography with a degree in Oriental History. He also occasionally participates in dramas such as guest starring in his fellow bandmate Yamashita Tomohisa's drama Kurosagi.