Original: 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.
Simplified: 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.