predecessor; senior official; senior member
Refers to a person who held a position or role before the current holder, or a senior member in an organization. Often used in workplace or military contexts.
I take over from my predecessor.
Please follow the instructions of the senior official.
後任者 means 'successor', the person who takes over a position after someone else.
Compound of 先任 (seniority, prior appointment) + 者 (person). The exact historical derivation is straightforward from the kanji components.