Flanders was born in Minneapolis, MN, the son of Bernice (née Brown) and Francis Michael Grey Flanders. Flanders began his acting career on Broadway before moving on to guest parts in television series. From 1967 through 1975, Flanders appeared in more than a dozen American TV shows, including six appearances on the CBS-TV series Hawaii Five-O (as six different characters). During this time, he was also prolific in TV movies. He also married actress Ellen Geer during this time; they later divorced.