According to the Oxford English Dictionary (2nd ed.), the first recorded occurrences of the relevant words were Tao 1736, Tau 1747, Taouism and Taouist 1838, Taoistic 1856, Tao-ism 1858, Taoism 1903 [clearly wrong, at least antedated by Balfour (1881)], Daoism 1948, Dao and Daoist 1971.
More generations of English speakers have learned about China through Wade-Giles (proposed in 1859, revised in 1892) than through Pinyin (approved in 1958, adopted in 1979).