A native of Brooklyn, New York, Yellen pitched in 14 games over the 1963 and 1964 seasons for the Houston Colt .45’s (now the Astros). At age 20, he made his big league debut on September 26, 1963, and held Roberto Clemente hitless in three at-bats. The following season he pitched in 13 games exclusively in relief, but had difficulty with major league hitters and was sent back to AAA. He pitched for two more seasons in the minors before retiring from baseball at the age of 22. Yellen attended Lafayette High, and he counted Sandy Koufax as a fellow alumnus. He pitched for Hunter College in New York and was named the team’s most valuable player two years in a row.