The short answer is yes. It's even possible to be mentored by an unbeliever about spiritual things. I was spot mentored recently by a Senior Chief who is not a believer and yet held me to the standard (as best as he knew how) that I should maintain as a Christian. I've also had a great mentor on my last ship who still advises me on issues pertaining to the military. He's not a believer either.
While it can be done to a certain degree, it is not the recommended solution to your mentoring problem. For one thing, an unbeliever cannot counsel you on matters of your faith. He can only hold you to the standard that you tell him you're trying to maintain. He can tell you what he thinks about your faith, or what you should do, but there is no reason to seriously consider it. After all, how in the world could he understand Christ's principles if he does not believe in Christ?
If you are looking for a mentor to fill a specific role, like teaching you how to be a good professional in your job field, then go for it. However, if you're looking for a good mentor who can help direct your life path, then you need a believer, and an experienced one at that.