In 1881, Tōyama became one of the founders of the Genyosha, a secret society and terrorist organization whose agenda was to agitate for Japanese military expansion and conquest of the Asian continent. The society attracted disaffected ex-samurai, and also figures involved in organized crime to assist in its campaigns of violence and assassination against foreigners and liberal politicians. In 1889, Tōyama and the Genyosha were implicated in the attempted assassination of foreign minister Ōkuma Shigenobu.
Immediately prior to the start of the First Sino-Japanese War, Tōyama organized the Tenyukyo, a secret society and paramilitary force that operated in Korea prior to the arrival of the Imperial Japanese Army, making detailed topographic maps, scouting out Chinese and Korean military installations and deployment, and arranging for logistic support. Along with Genyosha operatives in Korea and Manchuria, the Tenyukyo provided interpreters and guides to the regular Japanese army after their invasion.
Tōyama was a strong supporter of Japanese control over Manchuria, and joined forces with the anti-Russian Tairo Doshikai movement in 1903. He also supported the Chinese republican revolutionaries against the Qing dynasty and gave considerable support to Sun Yat-sen. When the Chinese revolution began in 1911, he went to China in person as an advisor and to personally oversee Genyosha activities and to provide assistance to Sun Yat-sen.
Following the Chinese revolution, Tōyama officially retired, and apparently refused to play an active role in the Black Dragon Society (Kokuryu-Kai) that he helped create as a successor to the Genyosha. He remained an influential behind-the-scenes figure in Japanese politics during the following years.