Publishing a paper that is not related your field I am graduate student in mathematics, but I did undergraduate in a different field (Astrophysics/Geophysics). I did my undergraduate reaserch somewhat seriously and have a good thesis as an undergraduate. My advisor told me that maybe the result I got is publishable. I have never done any publishing beforehead, so preparing for it may take some time. Of course by doing that, I will have less time to focus on mathematics. Also it is not guranteed that the result will be published in the end. 

Now the question is : will it be beneficial (in terms of career in mathematics) to spend time to prepare publishing my undergraduate thesis, which is different field from my current major? Will the publication make my CV stronger?

Of course I know it really depends on some factors such as 1.how much time will I loose by preparing the publication? 2.how is my undergraduate thesis related to the mathematics? 3.What is my future goal?, and perhabs more, so there is no single answer. I just want to listen to the opinions from more experienced.
My honest advise is to try to publish it.
Your work speaks for yourself. 
It means you were dedicated, motivated and got some results. 
If it is not some kind of 2 years writing thing, I would do it.
All your work represents you and your education and motivation, and I would always chose to work with someone who showed passion for some area, even if it is not his major.

P.S. In my country, mathematics and astrophysics are ojd the same faculty. They are connected.