Learn French: Il agit en tant que mentor dans cette fonction.

When using en tant que to express someone's profession, role, or capacity (meaning "as a" or "in the capacity of"), you generally drop the indefinite article (un or une) before the noun. That is why it is simply en tant que mentor and not en tant qu'un mentor.

Could I use comme instead of en tant que in this sentence?

Yes, Il agit comme mentor is also correct and means the same thing. However, en tant que is slightly more formal and specifically emphasizes the official capacity or status he holds, whereas comme can sometimes just mean "like" in other contexts.

Does the verb agir mean "to act" as in performing on stage?

No, agir means "to take action", "to behave", or "to function". Here, agir en tant que means he is operating or functioning in that specific role in real life. If you wanted to say he acts in a play or movie, you would use the verb jouer.

Why is fonction translated as "position" rather than "function"?

While fonction can mean a mechanical or mathematical "function", in a professional or corporate context it is the standard French word for a "position", "role", or "office" that someone holds in a company or organization.