Is it mandatory to govern a nation with Shariah law? If so, would that not make it impermissible to hold government positions in countries with secular law?
Sharia law is for Muslims who believe and obey Allah and not for non Muslims.
When Muslims want to implement the Islamic law which is known as Sharia law, it is their decision and no one can object on them as we don't object on people who opt to follow their own way of legislation in any part of the world. It is part of the basic rights to believe in your own faith and implement it.
Holding a government position in secular countries is permissible for Muslims as far as they don't indulge in injustice or sinful acts.
'Wassalam.