In 1866, he was ordered by his domain to go to Nagasaki, where he met Gotō Shōjirō and Sakamoto Ryōma, who gradually convinced him of the futility of attempting to preserve Japan's national isolation policy.

In 1869, he was appointed a junior councilor, and spent the next three years in Tosa, reforming the clan's government, while his fellow clansmen, Itagaki Taisuke and Gotō Shōjirō remained in Tokyo.

He spoke out strongly against the profligate spending of the Tosa delegation in Tokyo and enacted cost reduction measures, which enraged Gotō, who had Tani removed.

After returning to Kōchi in 1881, Tani was one of the founders of the conservative political party, Chūseitō, which was created to challenge Itagaki's Freedom and People's Rights Movement.

Tani was one of the leaders of a group called the "conservative opposition", an alliance of retired generals, politicians and peers who were critical both of the Meiji government and the movement for popular rights.

Leading members in the conservative opposition, including Tani, believed that the "nation" is composed of a mystical union between Emperor and people, and therefore the government should rest neither with the oligarchs nor with the political parties.

In 1885, Tani joined the first Itō Hirobumi cabinet as the first Minister of Agriculture & Commerce, however, he soon resigned over dissatisfaction with what he perceived to be the weak and vacillating foreign policy of Inoue Kaoru, especially with regards to the revision of the unequal treaties.

On the other hand, he was set against any concession to the foreigners in the struggle against the unequal treaties and believed Japan must refrain from alliances with great powers in order to uphold its own unique culture, tradition and national essence.

Tani's unique position as a rare anti-Imperialist thinker in the Japanese military establishment, earned him a description by Mutsu Munemitsu, Japan's foreign minister, as a "lone flower in a field of grass.