Born in Chicago and brought up on the Monterey Peninsula, Flavin received an AB in 1947 from Stanford University and an LLB, cum laude, in 1950 from Harvard Law School. He served in the U.S. Navy from 1943 until 1946 and again from 1951 until 1953. He was admitted to practice in the District of Columbia in 1950 and in California in 1952. Associated with Pillsbury, Madison & Sutro (now Pillsbury Winthrop) in San Francisco 1953 to 1967, he joined Mr. Ehrman in practice in Monterey in 1967 and co-authored with him a work on the taxation of California property.

He is a member of the Taxation and Estate Planning, Trust and Probate Law Sections of the California State Bar. He was recipient of the 2000 Joanne Garvey Award by the Taxation Section of the State Bar. Mr. Flavin continues to practice in Monterey, specializing in both property taxation and estate planning.