Can there be a hundred percent chance of getting mono if you kiss someone who had it?

Not really: A majority of adults had been exposed to eb virus (virus causing mono) during their childhood and developed an antibody to it.

Relative risk: Most are highly contagious for 3 weeks, some remain relatively so for 3 months while all can occasionally continue to shed virus for years.

Most people...: ...Had mono as small children without ever noticing it, and will never get it again. Those who did not, are at risk for catching it even without kissing.