Following his stint at sea, English physician Thomas Charles Byde Rooke hired Fornander as a coffee planter in Nuʻuanu Pali, Oahu.
When the paper failed in 1855, Fornander began a new venture called the Sandwich Islands Monthly, which was to cover both items of local interest and discussion of the great scientific, literary, and theological questions of the day.
In late 1863, the new Hawaiian king Kamehameha V recognized Fornander's talents and appointed him to the nation's privy council, which were thirty of the most distinguished men in the islands.
While undertaking these duties, Fornander had long been developing theories of Hawaiian origins and collecting material for a work setting out his thoughts.
In 1877, he finished the first volume of his monumental An Account of the Polynesian Race, its Origin and Migrations, and the Ancient History of the Hawaiian People to the Times of Kamehameha I, which was published in London in the following year.
This volume dealt with the now-discredited hypothesis that the Polynesians were Aryans, both racially and linguistically, who had over the ages migrated through India and the Malay Archipelago into the Pacific islands.
When expelled by Melanesians, the inhabitants made their way to Samoa and Tonga, and by AD 400 or 500, to Hawaii, where they lived in isolation until the 11th century, when new groups began to arrive.
Fornander paid special attention to legends and genealogies that he thought preserved the history of the Hawaiian islands after their settlement—their external and internal wars, dynastic quarrels, and eventually their contact by Captain James Cook and George Vancouver.
He was invited to become a corresponding member of the California Academy of Science in 1878, and in the following year the Hawaiian King made him a Knight Companion of the Royal Order of Kalākaua.
Fornander thrived on the international praise that his work had won him, but continued in his various official duties, including serving as acting governor of Maui.
These have Fornander's transcriptions in Hawaiian, together with a later English translation edited by Thomas George Thrum on facing pages, and were published from the years 1916 to 1920.