"Rusty" wrote
Mariner 4 (Mariner-Mars 1964) used solar pressure vanes at the end of
its solar panels in an attempt to use the solar wind for attitude
control. It wasn't solar sailing, but it may have been the first
attempt at using the solar wind by a spacecraft.

David Portree also mentioned this on NASAWATCH, and it's a very good catch!

MCO on its way to Mars in '99 had only a single solar panel and it
was so asymmetric that it was an unintended 'solar sail' and this
contributed to the nav error that eventually doomed the probe.