Complex sentence: The series was originally emceed by Johnny Carson and announced by Bill Nimmo; partway through the run, Nimmo was replaced by Ed McMahon, and from that point until 1992 the two would spend the majority of their careers together.
Split into simpler sentences: The series was originally emceed by Johnny Carson and announced by Bill Nimmo. Partway through the run, Nimmo was replaced by Ed McMahon, and from that point until 1992 the two would spend the majority of their careers together.