Will Marijuana Possession be permanently marked on my record?

A few years ago, I was caught with a little bit of marijuana in my possession ($15-$20 worth) in my car. Once the cop found it, she basically wanted me to stomp it out and she would let us go. Little did I know, the friend I was with at the time had about half a zip of weed on him so we ended up getting written up a ticket as a misdemeanor which eventually led to a court date in which I plead guilty but since it was a minor offense all I had to do was go to about 6 months of AA and NA meetings while meeting up with a counselor once a month and pay a fine with no jail time at all. The last time I talked to my lawyer, he said that since it isn't that big of an offense it will eventually be expunged off my record after a certain time, if anything, but I don't know if it's true. I have given up marijuana and have never gotten into any kind of trouble since then. I am currently pursuing a career in the fire service but I'm worried that when they do a background check, they might disqualify me based on that. Should I be honest about it or treat it like it never happened?