"taken office" is correct and usable in written English.
You would use this expression to describe when someone officially assumes a position of authority. For example, "In January, the new mayor took office."
Since he's taken office, the threats of violence have continued.
It's been about a month now since I've taken office.
He had just taken office when Crown Heights exploded in rioting against Orthodox Jews.
By year's end four more members had been approved and taken office.
Having taken office just six weeks ago, he has lost no time in implementing these plans.
But since he has taken office, the actions of his Administration have indicated otherwise.
Her husband had taken office with a pledge to dismantle the old system.
Fast forward 20 years: Clinton has been elected president but not taken office.
She should have taken office last month following an oath-taking session on 12 October.
No sooner had he taken office than it was deemed unsafe.
Either way, the first test of Mr Peña's mettle has come even before he has taken office.
