Will one old spark plug make the check engine light come on in a 93 F-150?

Yes, one defective spark plug can cause the check engine light to come on. That one plug can be not firing properly and this would cause a miss on that cylinder. That miss would cause the ECU to set a fault code. The code will tell you what component is being effected. My advice is to replace that plug. Always replace plugs as a set, and never leave one old plug installed.