Hey Selso! I play the mandolin and I use a Snark SN-2 tuner. It works really well, it's chromatic, and it tunes all instruments. It's not to expensive and a great device. I play about 12 different instruments and for every one that is stringed I use the Snark tuner...it's the best tuner I've ever had and is really popular. If you want to check it out, it's the second one on this link.
http://www.snarktuners.com/clip-on
Mine wasn't that expensive; I bought it at my local music store.
A guitar is tuned differently (EBGDAE) than a mandolin (EADG) so no, it wouldn't work. But you're not an idiot who did something wrong!
Good luck!!