A prominent developer at Nintendo, Yamagami joined the company in the late 1980's, where he joined the R&D department under legendary Nintendo producer Gunpei Yokoi. He started working as a programmer for the various Gameboy titles which were being developed at the time, but his superiors recognized his ability and he soon became a lead designer for many Famicom titles. As his experience grew, Yamagami soon became Nintendo's lead developer for puzzle games, developing titles such as Tetris, Yoshi's Cookie, Dr. Mario and many others. Later on, Yamagami was partnered with Intelligent Systems and with their help he created a new type of puzzle game in Panel de Pon series.
During his time at R&D1, Yamagami also began coordinating with other development teams on their own games. Of all the external teams, he most famously worked with Intelligent Systems on their flagship series, Fire Emblem, a relationship which continues to this very day. Yet despite working with other teams, he still was able to lead his own internal team, and became the lead designer for the Game & Watch Gallery series.
After Nintendo CEO Satoru Iwata restructured the development teams in 2004, Yamagami was made the head of Software Planing & Development 2, a studio which focuses exclusively on working with teams outside of Nintendo such as Monolith Soft, HAL, Ganbarion and others. Continuing from Fire Emblem, Yamagami also oversees Nintendo's largest RPG series, Pokemon, starting with Pokemon Ruby/Sapphire for GBA.