Can a 10-speed chain be used with a 9-speed cassette? I have an extra 10-speed chain I would like to use with a 9-speed cassette on my mountain bike. Will this combination work ok?
The 10 speed chain is narrower and will work. Not ideal and the shifting might not be as crisp.

Since you have an extra chain you can use it, but if purchasing new I would get a 9 speed chain.