I tried to draw up an accurate Corleone family Boss timeline once. I figured about the following. Sorry my years aren't quite accurate, I'm just reprinting this from memory.

Vito Corleone (1919/1933 - 1950)
- Santino Corleone (Acting Boss 1947? - 1949?)
Michael Corleone (1950 - 1979/1997 not sure if he steps down or just goes until he dies)
- Tom Hagen (Acting Boss 1958 - 1959)
- Nick Geraci (Acting Boss, depends if you want to count him)
Vincent Mancini (1979/1997 - present)