Kauai and Niihau had eluded Kamehameha's control since he first tried to add them to his kingdom in 1796, a year after Kaumualii became ali'i nui. At that time the governor of Island of Hawaii led a rebellion against Kamehameha, forcing him to return home. He tried again in 1803, but disease ravaged his armies, and he called a retreat to heal his men and work on his strategy. Over the next years Kamehameha amassed the largest armada Hawaii had ever seen – foreign-built schooners and massive war canoes, armed with cannons and carrying his vast army. Kaumualii decided to negotiate a peaceful resolution rather than resort to bloodshed. The move was supported by Kamehameha as well as the people of Kauai and the foreign sandalwood merchants on the island, whose trade was hurt by the constant feuding. In 1810, Kaumualii became Kamehameha's vassal, and Hawaii was united under one sovereign for the first time. Kaumualii continued to serve as Kamehameha's governor of Kauai.
Kamehameha died in 1819, and the Hawaiians grew fearful that Kaumualii would sever Kauai's relationship with the united Hawaii. Kamehameha's widow Elizabeth Kaahumanu, who exerted much political force in kingdom, kidnapped Kaumualii and forced him into marrying her, ensuring the island chain's union remained unbroken during the reigns of Kamehameha's successors. They remained married until his death in 1824.
Kaumualii was popular both among his people and the foreigners who visited and worked on his islands. Captain George Vancouver, who had given the young king a flock of sheep as a gift, was thanked with a lavish banquet and described his host glowingly. Kaumualii was described as handsome, likeable, and courteous, as well as a capable leader.
King Kaumualii's granddaughter Alii Kapiolani (1834 – 1899) of Hilo (also known as lady Esther Kapiolani or Julia Kapiolani, eldest daughter of Kaumualii's daughter Princess Kekaulike Kinoiki) married a certain Alii David Kalakaua, postmaster-general of the Kingdom of Hawaii. In 1874, the couple was elected by the Hawaiian legislature as King and Queen of the Hawaiian Islands, starting to reign as king Kalākaua I and Queen Kapiolani. Her youngest sister Princess Victoria Kuhio Kinoike Kekaulike (1843 – 1884) of Hilo was later elevated as Governor of the island of Kauii, Princess and Royal Highness. Her other sister Princess Virginia Kapo'oloku Po'omaikelani (1839 - 1895) succeeded her sister as Governor of the island of Kauii and was made Guardian of the Royal Tombs.
Princess Victoria Kuhio Kinoike Kekaulike progeny are called the House of Kawananakoa and are regarded as "legitimist" claimants to the crown of the now defunct Hawaiian monarchy. Her children included:
Only David had issue.