Kaumuali‘i was the only son of Queen Kamakahelei and her husband, Aliʻi Kāʻeokūlani (Kā‘eo;) he was born in 1778 at Holoholokū, a royal birthing heiau specifically designated for the birth of high ranking children.
When Vancouver was anchored off Waimea, Kauai, he became interested in Kaumuali‘i, who was then about twelve years old. Vancouver found the child quiet and polite and good-tempered. He was interested in the new things which he saw, and asked intelligent questions.
When Vancouver made his second visit, he brought sheep as a present to the young chief. Kaumuali‘i entertained him with a dance of six-hundred women.
Kaumuali‘i kept up his interest in foreigners. They were his friends and taught him to read and write. Kaumuali‘i sent his son Humehume (Prince George) to America to be educated. (The young Prince later returned to the islands with the first party of American missionaries, in 1820.)
Kaumuali‘i became ruling chief of Kauai upon the death of his father Kā‘eo.
In 1784 Kamehameha I began a war of conquest, and, by 1795, with his superior use of modern weapons and western advisors, he subdued all other chiefdoms, with the exception of Kauai.
King Kamehameha I launched his first invasion attempt on Kauai in April of 1796, having already conquered the other Hawaiian Islands, and having fought his last major battle at Nu‘uanu on O‘ahu in 1795.
Kauai’s opposing factions (Kaumuali‘i versus Keawe) were extremely vulnerable as they had been weakened by fighting each other (Keawe died and Kaumuali‘i was, ultimately, ruler of Kauai and Ni‘ihau.)
About one-fourth of the way across the ocean channel between O‘ahu and Kauai, a storm thwarted Kamehameha’s warriors when many of their canoes were swamped in the rough seas and stormy winds, and then were forced to turn back.
Kamehameha’s second attempt was thwarted, again, when an epidemic, thought to be typhoid or dysentery, swept through the population, killing thousands. The sickness delayed for a second time Kamehameha’s goal of conquering Kauai.
In a renewed effort for a large-scale attack on Kauai, Kamehameha began assembling a formidable armada of sailing ships in Waikīkī, using foreigners to construct the vessels. The invasion never took place.
In the face of the threat of a further invasion, in 1810, at Pākākā on Oʻahu, negotiations between King Kaumuali‘i and Kamehameha I took place and Kaumualiʻi yielded to Kamehameha.
The agreement marked the end of war and thoughts of war across the islands. Although Kaumuali‘i had ceded Kaua‘i and Niʻihau to Kamehameha I, he generally maintained de facto independence and control of the island following his agreement with Kamehameha.
It is believed that in 1816 Kaumuali‘i considered it possible for him to claim rule over Kaua‘i, Ni‘ihau, O‘ahu, Maui, Moloka‘i and Lāna‘i, if he had Russian support. The Russians meanwhile were searching compensation for lost trade goods, as well as expanded trading opportunities.
Kaumuali‘i and Russian representative Georg Anton Schäffer had several agreements to bring Kauai under the protection of Russia, as well as weapons and ammunition from Schäffer, in exchange for trade in sandalwood. While agreements were made, subsequent battles never took place.
After King Kamehameha I died in 1819, Kaumuali‘i pledged his allegiance to Liholiho, Kamehameha’s son and successor. In 1821, Liholiho (King Kamehameha II) anchored his royal ship Ha‘aheo o Hawai‘i (Pride of Hawai‘i) in Waimea Bay, and invited Kaumuali‘i aboard.
After boarding the ship Kaumuali‘i was effectively taken as a prisoner and the ship sailed for O‘ahu. Kaumuali‘i settled in Honolulu and became a husband of Ka‘ahumanu, widow of Kamehameha I.
Hiram Bingham was on a preaching tour of the island of Kauai in 1824, shortly before King Kaumuali‘i died. Kaumuali‘i had been living on Oahu for three years. Bingham spoke to him just before coming to Kauai.
Bingham writes: “We found Kaumuali‘i seated at his desk, writing a letter of business. We were forcible and pleasantly struck with the dignity and gravity, courteousness, freedom and affection with which he rose and gave us his hand, his hearty aloha, and friendly parting smile, so much like a cultivated Christian brother.”
When the king died, Bingham said a gloom fell over Kauai. Kaumuali‘i was buried at Waine‘e Church (Wai‘ola Church,) on Maui.
After Kaumuali‘i’s death his son Humehume tried to seize the throne by leading a rebellion on Kauai, but he was defeated and sent to O‘ahu, where he could be watched.
King Kaumuali‘i’s granddaughter Kapiʻolani (1834–1899) married King Kalākaua.