How Good Is A Recommendation Letter For MS in CS From A Workplace If It's Coming From A Mentor? I have 3 years of work experience in cloud computing and virtualization and am intending to illuminate this in my SOP. With what degree of credibility will the above mentioned work experience be accepted by admissions committees if I do not have any letter from my workplace at all? Also, since it is difficult for me to get a recommendation from my immediate manager at my workplace, if at all I do get one from my company, it may be from a colleague who was my mentor (no managerial role). How good will such a recommendation be? I already have two recommendations from my college professors, each of which highlight one of my two research publications. It is for the third recommendation that I have the choice between this letter from my mentor at my workplace and from an assistant professor at my college who had taught me in a number of courses and labs. Which recommendation should I go forward with? Please advise on these matters.
Unless the MS is very applied, I'd think that an additional academic recommendation would be better. That is especially true if they can speak to different aspects of your work. 

But a letter from a mentor should, in general, be fine. It might even be better than one from a manager as the person likely has more knowledge of you and your contributions. The letter from a manager is likely to be a bit pro forma. The recommender should be clear about the relationship (i.e. mentor, not manager).