Hiram Kahanawai was born on September 1837, at Luaʻehu, Lahaina, on the island of Maui, to Kaʻaha and Kamaile. His paternal grandmother Kahikaheana was a trusted kahu (caretaker) who served the family of High Chief Kalaʻimamahū, a brother of King Kamehameha I, and his daughter Kekāuluohi. During the Great Māhele of 1848, his father received the land allotments on the islands of Oahu, Maui and Kauai including half of the ʻili of Kawananakoa, in Honolulu, the ahupuaʻa of Kaulalo, near Lahaina and the ʻili of Kuiloa, near Hanapepe. Kaʻaha died before 1852 and left all his landholding to his son. On May 29, 1852, Kahanawai was placed under the guardianship of his brother-in-law William Luther Moehonua, the husband of his sister Lucy Muolo. He was also a relative of Queen Emma of Hawaii, by a junior line of descent, and distant relative of the family of Kalākaua and Liliuokalani.
During his youth, he continued his family kuleana (responsibility) in serving the Hawaiian royal family. He served as the chief steward and butler in the royal household under King Kamehameha IV and his wife Queen Emma from 1855 to 1863. On October 15, 1853, Kahanawai had been appointed by the king, while he was heir-apparent to Kamehameha III, as First Lieutenant of the Infantry in the Royal Hawaiian army. After Kamehameha IV's death, he continued to serve the household of the Queen Dowager Emma during her widowhood.
On March 20, 1855, he married Virginia Kapoʻoloku Poʻomaikelani (1839–1895), a trusted lady-in-waiting and household attendant of Queen Emma. The couple did not have any children of their own but adopted and raised Edward Abnel Keliʻiahonui (1869–1887), the nephew of Poʻomaikelani.
Sometimes prior to 1873, Kahanawai and Poʻomaikelani left the service of Emma to join the factions loyal to Kalākaua. In the election of 1874 Kalakaua defeated Queen Dowager Emma to succeed Lunalilo as the monarch of Hawaii. Besides his own distant family ties to the new dynasty, Poʻomaikelani's elder sister was Queen Kapiolani, the wife and consort of the new king. After Kalakaua restored the Hawaiian army which had been disbanded by his predecessor, he appointed Kahanawai to the rank of Captain and in the capacity of Commander of the Household Troops.
Kahanawai, who had been in poor health for some time, died suddenly of a heart attack, on August 7, 1874 while in an audience with the King. His obituary reported his sudden death:
On Friday last (Aug. 7) Captain Hiram Kahanawai died very suddenly as it is supposed from aneurysm. He had just entered the King's presence, and bowed to His Majesty, when he dropped on the floor, and was carried out dead. He was a true and faithful servant, of a quiet and reserved disposition, and made a favorable impression upon all who knew him. His funeral was attended on Saturday afternoon from St. Andrew's Chapel, and his remains escorted to Nuuanu cemetery by the infantry troops and numerous personal friends, including members of the royal family. Among Hawaiians there are few who have left a more pleasant memory
On September 17, 1873, Queen Emma had noted in a letter to her cousin Peter Kaʻeo that Kahanawai was suffering from "Anurism", and although still resenting his betrayal, prayed for the recovery of her former servant for the sake of her dead husband and son. According to a later letter from Kaʻeo to Emma, he told her that both he and Jonatana Napela suspected that Kahanawai had been poisoned and the death had been covered up by the government. According to historian Alfons L. Korn, "No criminal charges were ever brought against anyone for being implicated in the death of Hiram Kahanawai."
He was buried at the Oahu Cemetery in Honolulu. His widow continued to be known as Mrs. Kapoʻoloku Kahanawai until she was created a Princess of the Kingdom by Letter Patent in 1883.