Usage patterns
〈人〉が主任になるfor someone to become the chief/head (of a section)
Ms. Tanaka will become the section chief starting next month.
〈人〉を主任に任命するto appoint someone as chief/head
The company appointed him as the new chief.
主任として〈動詞〉to do something in one's capacity as chief/head
As the team leader, she keeps the team organized.