Masatatsu Yonemura was born to Japanese parents in Riverside, California on October 7, 1916, the sixth of eight children. His parents, Tameki Yonemura and Kiku Imamura, left Kumamoto prefecture on Japan's southern island of Kyushu and went to work in Hawaii around the turn of the twentieth century, then immigrated to Southern California around 1906.

With the encouragement and support of his older sister Yoshiko, Mas and his younger sister, Aiko, became the first in their family to graduate from college. Mas first attended Riverside City College and then transferred to UCLA where he received his BA in Economics with highest honors in 1939. Although initially interested in engineering, Mas decided instead to enter law school and received his law degree from UC Berkeley's Boalt Hall in 1947.

During his last semester of law school Mas was drafted into the U.S. Army. He served as a translator of technical documents in the U.S. Army Military Intelligence Service in the Pacific during WWII -- while his parents and siblings were interned in Poston, Arizona for their Japanese heritage.

In 1943, Mas married Toshiko Furuta in Joplin, Missouri. Just a few months before, Toshiko had won her own release from Poston. After the war, Mas was briefly stationed in Tokyo during the first months of the U.S. military occupation of Japan, and later returned to Berkeley, California, to complete law school. He and Toshiko settled in Berkeley, where they raised their family of three children - Ann, Jean, and Paul. They were married for 59 years until Toshiko's passing in 2002.

Mas began his nearly 60 year career in law at Vaughns & Berkley, an African-American firm located in West Oakland. After seven years there, he opened his own office in downtown Oakland, later expanding to a partnership with the late Joe Yasaki and finally adding partners Ken Kawaichi and Bob Baizer. Over the years, Mas practiced in many areas of law and was the attorney for many San Francisco Bay area Japanese-American families, farms, nurseries, and businesses. Starting in the late 1950's, Mas also became an attorney for several major Japanese companies seeking to do business in the U.S. including Japan Airlines and Japan Foods. This allowed Mas to travel frequently to Japan, where he made many friends and cultivated a true appreciation and passion for Japanese art, especially modern Japanese prints.

Mas's law practice increasingly focused on immigration law. One of the aspects he found most rewarding about this work was the unparalleled opportunity to meet people from all over the world and from all walks of life. In many instances, Mas's concern for justice for the Japanese-American community coincided with his work in immigration and nationality law. The National Center for Immigrants' Rights in Los Angeles noted that "…as a pro-bono lawyer, he assisted the late Wayne M. Collins, Esq., in representing a number of Japanese-Americans who 'renounced' their U.S. citizenship while they were involuntarily interned during World War II. He also restored the citizenship rights of a number of Japanese- Americans who, while possessing dual nationality, were stranded in Japan during the war and presumed expatriated under the nationality law of 1940 because of service in the Japanese Army as conscripts, teaching in the public schools of Japan, or having voted in the first post-war elections during the U.S. Occupation of Japan."

Mas also served as president of the American Immigration Law Foundation (AILF) and as a board member of the Alameda County Bar Association, the Boalt Hall Alumni Association, and the American Immigration Lawyers Association (AILA).

As a Nisei (U.S.-born Japanese-American) and as an attorney, Mas actively supported many organizations and individuals in the Japanese-American and Asian-American communities, as well as others who were fighting for social justice, such as California farm workers.

Among the many honors earned in his distinguished career, Mas was decorated by the Emperor of Japan with the Order of the Sacred Treasure, Gold Rays with Neck Ribbon for his work in bettering bilateral relationships between the United States and Japan through a better understanding of the laws of each country.

Besides his career and family, Mas had a great love of Japanese gardens and gardening, automobiles and road trips, collecting modern Japanese prints, and traveling, especially to Japan. He was also an avid reader and was well versed on many topics. For his hard work and passion, he will be greatly missed.