How do you get mold smell out of jeans?

Hot water wash with laundry detergent that has a scent to it. If it doesn't come out then those jeans are too gross to wear. :-)

Were the jeans exposed to mold while, say, gardening, or were they packed away in a damp basement for years? In other words, what's the reason for the mold smell?

If you have a programmable washer, use hot water, place vinegar in the prewash dispenser, borax powder in the main wash and add an extra rinse. If you have a standard washer, run with vinegar in the wash water, rinse, then use borax in the next wash cycle, and finish with a plain water rinse.

Some mold in clothing can't be removed even after repeated washings. If you've taken WG's advice and still have the mold, then you should toss the jeans as they are too riddled with the type of mold that will only keep coming back and spreading to other clothing when either stored with the clothes or washed with the unmoldy clothes. Saw this happen first hand with some moldy clothes and they ended up contaminating the new clothes in a week and finally the original clothes had to be disposed of.