I already landed an agent and worked happily with her for over three years. Sadly, she had to leave the biz, and I have to dive back into the ocean. Is it better to mention that fact in my query, and if so, where and how? I'd like prospective agents to know that someone already thought I was ready for representation and is willing to vouch for my ability and professionalism.The question has two possible answers: yes and no (that's a lot of help, right?)
Yes, you need to mention it IF this novel you're querying went on submission or sold.
No, you don't need to mention it if your novel did NOT go on submission.
Remember, an agent is looking for work to sell, so a project that's made the rounds is less attractive than one which is fresh and new.
If you're published and fulfilling a contract, that commission goes to the previous agent which means a new agent will be working for free for a while.
If your work didn't go on submission, you don't need to tell anyone about your previous agent.
If your work did, you put the information in the housekeeping section of the query (paragraph 3.)
You phrase it as: I was previously represented by Felix Buttonweezer who has left the business.
Your desire to let someone know that another agent found you ready for representation, and is willing to vouch for you is a lovely idea, but it's like getting your first wife to write your new Tindr profile.
You will convey all those things in your lovely query, and any subsequent conversations. You never need mention it.