"总理之职" can be translated as "the position/post of Prime Minister" or "the role of Prime Minister" in English, depending on the context.
If emphasizing the official title or position within the government structure, "the position/post of Prime Minister" is more appropriate.
If focusing on the duties or responsibilities associated with the role, "the role of Prime Minister" could be a better fit.
For example:
He assumed the position/post of Prime Minister last year. （他去年就任总理一职。）
She fully understands the role of Prime Minister and the challenges it entails. （她完全理解总理这一角色的职责以及所面临的挑战。）