General:
* Assume control
* Seize control
* Gain control
* Acquire control
* Dominate
* Control
* Rule
* Govern
* Prevail
* Occupy
* Capture
* Subdue
* Conquer
Business/Corporate:
* Acquire
* Buy out
* Merge with
* Absorb
* Consolidate
* Amalgamate
Personal:
* Take charge
* Step in
* Assume responsibility
* Take the reins
* Take the lead
Informal:
* Run the show
* Take the wheel
* Call the shots
More specific synonyms:
* Overthrow (replace by force)
* Inherit (take control through succession)
* Usurp (seize power illegally)
* Supplant (replace)
* Displace (remove from a position)
The best synonym depends on the context of the sentence and the specific meaning you want to convey.