Namioka was born in Beijing, the daughter of linguist Yuenren Chao and physician Buwei Yang Chao. The family moved often in China. In 1937, the Chaos were living in Nanjing, and fled westward in the face of the Japanese Invasion. They eventually made their way to Hawaii, then Cambridge, Massachusetts. Namioka attended grade school in Cambridge and excelled at mathematics.

Namioka attended University of California, Berkeley, where her father was a professor of Asian Studies. Here she met and married Isaac Namioka, a fellow graduate student in mathematics. The Namiokas moved to Ithaca, New York, where Isaac Namioka taught at Cornell University, and Lensey Namioka taught at Wells College.

In 1959, the Namiokas' first daughter Aki was born, followed by a second daughter Michi, who was born in 1961. The family moved to Seattle in 1963, when Isaac Namioka accepted a position at the University of Washington.

In the 1970s, on a visit to Japan, Namioka visited Namioka Castle. The experience inspired her to learn more about the samurai. This study culminated in The Samurai and the Long-nosed Devils, which was published in 1976. Namioka expanded this book into a whole series of books about samurai. Namioka also wrote a series of books about a Chinese American family named Yang, and several books about young women and girls facing difficult choices.

Lensey Namioka is the only person known to have the first name "Lensey." Her name has an especially unusual property for a Chinese person born in China: there are no Chinese characters to represent it. Lensey's father, Yuenren Chao, was cataloguing all of the phonemes used in Chinese. He noted that there were two syllables which were possible in the Chinese language, but which were used in no Chinese words. These syllables could be written in English as "len" and "sey." His third daughter was born soon after, and he named her "Lensey."