Legal Question in Civil Litigation in California

Maybe. It would depend upon the circumstances. There might not be a conflict or, if there is, it might be one that the client and/or the other parties can waive.

There is no black and white answer to that broad of a question. There are statutes and professional standards regarding mediator conduct, and there are detailed statutes and rules of professional conduct for lawyers, that are implicated by your question. Without reviewing the actual specific facts and circumstances of the particular situation, there is no way to answer.

If the mediator learned non-public information about the side he is now in opposition to, then there would be a clear conflict of interest.

If the lawyer mediated between A and B and is nopw representing A against B, then there is a huge conflict of interest. The lawyer must be disqualified from such representation and reported to the State Bar for disciplinary action.

I disagree with Mr. Tashjian. He seems to presume that the mediation involved the same dispute (or at least a related one) as the subsequent lawsuit. That is not necessarily true. If the disputes are different and only the parties are the same, it is likely that the mediator would not have a conflict. As I said earlier, though, the answer will depend upon details which you have not provided. You should discuss your situation with a lawyer as soon as possible.