Depends on your interpretation or definition of Karma. If, like a traditional Buddhist, you believe that every cause has an effect, than yes, as an agnostic I firmly believe that. It is, in fact, a widely held belief among most belief systems. Among Islam, Judaism, and Christianity, there is the "reap what you sow" mentality. For strict Buddhists, that is all karma is.
If, on the other hand, you believe that karma means that if you do something bad, then something bad is going to happen to you, then I would say no. Case in point, many politicians and many of the uber-wealthy consistently do things that someone would deem as "bad," yet, they continue on with the status quo. This would mean that the notion of karma is not accurate.
If, however, you believe that ULTIMATELY what you do in your life adds to your overall contentment and/or suffering, then karma is the reason.
Just a thought,
SOP