Yamaguchi described his experience at Expo ’70 as eye opening, motivating him to shift gears, and leading him to take up the medium of video, with which he had begun experimenting in the late 1960s, in earnest. In 1972 he co-founded the Tokyo-based video collective Video Hiroba alongside Fujiko Nakaya, Hakudō Kobayashi, Nobuhiro Kawanaka, Masao Kōmura, Toshio Matsumoto, Rikurō Miyai, Shoko Matsushita, Yoshiaki Tōno, Keigo Yamamoto, Sakumi Hagiwara, and Michitaka Nakahara, among others. With this group he developed what art historian Jung-Yeon Ma describes as “video as a social medium,” focused on the concept of communication through both experimental group exhibitions and community projects. Aside from the group activities of Video Hiroba, Yamaguchi also began making video installations, such as his "Las Meninas" and "Video Landscape", both shown at the Bienal de São Paulo in 1974. By the late 1970s Yamaguchi was developing his concept of the "Imaginarium" that would serve as inspiration for his environmental sculptures of the 1980s and ‘90s. Aside from his work as an artist, he also began teaching as a professor in the School of Art and Design at University of Tsukuba, where he remained until 1992.