A funny thing about that Lalo Schifrin theme for the M:I series is that
it wasn't even intended as that. It was just a throwaway cue during a
chase scene of the pilot episode (now available on DVD). Getting
strapped for time, the producers talked Schifrin into rearranging that
one little bit that they liked, and making it the series theme.
There's a good explanation of all of this in Herbert Solow's and Robert
Justman's book about Star Trek. They were the producers of both series,
both debuting the same season from Desilu.
Watching that first season a couple of months ago, I enjoyed seeing a
bunch of the familiar ST guest stars pop up on M:I in other roles. Even
some of the sets are recognizable; Desilu was a small company doing both
shows at once, with tight space. Another fun connection is that Leonard
Nimoy went over to join the cast of M:I for several seasons after ST ended.
Obligatory harpsichord content: Desilu wheeled one of those into the
studio during that same first season, 1966-7:
It even has casters!