In 1957 , Mitropoulos and Leonard Bernstein served together as Principal Conductors until , in the course of the season , Bernstein was made conductor , becoming the first American-born-and-trained conductor of the Philharmonic . In 1957 , Mitropoulos and Leonard Bernstein served together as Principal Conductors until , in the course of the season , Bernstein was appointed Music Director , becoming the first American-born-and-trained conductor to head the Philharmonic .