Depends on the saddle whether you want to risk bleach on it, I guess. A 10% solution is what they usually recommend for mold. Wipe off all visible mold, spray it with the bleach solution, let it set for 15-20 minutes, wipe with a damp cloth.

I've done it with my husband's leather shoes and with my own motorcycle saddle, with no ill effects. I probably wouldn't use it on very dry leather (although I would think you could wipe off visible mold, treat the leather and then bleach). Personally I don't leave anything mold-prone in the garage if it's small enough to bring into the house.