Rathmann was born on December 25, 1927, in Milwaukee, Wisconsin. His mother was Edna Blatz, from the family who ran the Valentin Blatz Brewing Company. His father was Louis Rathmann, who worked in various local businesses. He was drawn to science and was inspired by an older brother and brother-in-law who were chemists, and by Mr. Leker, his high school chemistry teacher at Milwaukee University School (where his grandfather had taught years before). He had originally planned to attend medical school, before switching to physical chemistry and obtained a B.S. degree in physical chemistry at Northwestern University in three years, then earning a Ph.D. at Princeton in 1951, by which time he had already been recruited by 3M as a research chemist, where he worked for twenty-one years, helping develop Scotchgard and rising from scientist to corporate manager.