Originally from Kashima han in 1834, he was sent by the domain government to Edo in 1855, to become a student at the Shoheizaka gakumonjo. In 1859, he returned to Kashima along with the sankin kôtai entourage of daimyô Nabeshima Naoyoshi; the following year, he was granted a post within the domain government, as Nabeshima's private tutor. In 1866, he was named House Elder (karô). Three years later, he became a grand councilor (daisanji), and a direct assistant to daimyô Nabeshima Naoyoshi.
After Nabeshima became governor of Okinawa, he sent a letter to Hara, requesting the aid of his longtime tutor and advisor in administering the prefecture. Hara then joined the prefectural administration in May 1879, and continued there for just over two years, leaving Okinawa in September 1881 along with Nabeshima. Hara then joined the House of Lords in 1890, but died just four years later, in 1894, at the age of 61.
Following Hara's death, his personal possessions were divided up. A portion of his collection of books and documents was obtained by the University of Ryukyus Library in 1988. The Hara Tadayuki Collection now held by the university includes numerous personal letters, poetry, photographs and government documents.
- Plaque on display at University of the Ryukyus Library.