What are the pros and cons of an Executive Director of a
not-for-profitorganiation in the United States sitting on it's Board
of Dirrectors in an Ex-officio
position?

One more chance to keep an eye on the skunks

- bein' there saves bugging the boardroom

Downside of being there is that it inhibits the little darlings.

- perhaps the optimum is to put the place under total surveillance,
deliberately allow them a pre-meeting and a post-meeting.
That way you can identify your putative successor before it becomes too late.

If there was a previous Executive Director, did that ED serve as an
ex-officio member of the board?  As the ED you would probably sit in
on a number of those board meetings anyway and only be ask to step out
on Executive Committee votes, etc.  The board is of course your boss,
so the idea that you can really keep an eye on the skunks is off the
point. Has it been suggested that you serve as an ex-officio or is
that something you have come up with on your own?  It can be
considered a  political move if you push the idea, so avoid that and
have a supporter on the board recommend you. It never hurts to know as
much as possible about what is going on in your shop.