Should I publish work from my undergrad now that I am in graduate school and no longer working in the same field? During my undergrad (past 3 years) I worked in a niche ecology lab and during the time generated some data. The work I did was not exactly groundbreaking but I think it was solid publishable work and could find a home in a decent journal. 

My PI is encouraging me to write up my work and submit it for publication in the next few months but I have just started graduate school and am working in an entirely different field (Biophysics with a sprinkle of cell biology). 

Considering that I expect writing up the work for publication will be a significant undertaking (seeing how it would be my first time writing a paper for publication), and the disparity between the subject of the paper and my current/future research, would it be worth the time commitment to follow my PIs advice and write the paper? Would there be any real benefit for my career in a different field if this work does get published?
The experience of having gone the whole way through the publication process is very valuable and will benefit you when you start writing up your new research.  In the long run (say by the time you'd be applying for tenure-track jobs) having a publication in another field won't help your CV much because people are looking at your research program.  But in the medium-term if you apply for postdocs you may not have many (or any) actual publications (given the time it takes to write-up and submit research) and it's going to make you look better because it shows to potential supervisors that you're responsible and capable of completing a project.