I've been doing a bit of research on Catherine, as I saw her recent shout-out looking for new work, and the agency has such a good reputation. From what I've seen on querytracker, she is requesting partials from a good percentage of queries, so obviously actively working on building a list.
She doesn't seem to have been an agent before. She shows up in a Google Books search being thanked in a few book acknowledgements from 1999 to 2001. It looks like she worked at Scribner at one point (thanked in 'About Town' a book about the New Yorker by Ben Yagoda) possibly as an editor's assistant. In the acknowledgements for a book called "The Secret Parts of Fortune" (2000) it looks like she was working for the agency The Robbins Office at the time. Then nothing for ten years or so.
She is the daughter of the writer Abigail Thomas, who's also been a book editor and agent. So a family business connection for sure. Perhaps she was an intern or something similar at Darhansoff & Verrill back around then (2001 or so).
I'm hoping that her relative newness might be offset by both the agency's reputation and contacts, and her own connections, if they're not out of date.
I sent a query on a novel as part of a new batch of submissions recently and got a request for a partial a few days later. The full is already out with one other agent, but it's been 9 weeks and I'm getting restless and thought I'd better start adding to my list of possibles, and I'd had my eye on the agency since they rep Neal Stephenson, who is amazing (not to mention Annie Proulx and Stephen King).