How to address an Oxford/Cambridge full-time lecurer who is not a Ph.D.? I am currently writing an e-mail to a member of Oxbridge staff. He is a "lecturer," full-time staff, who co-ordinates a well-known program, but does not hold a PhD, only an MA. 

I can't call him "Dr. ," or even "Professor ," so is "Mr." sufficient? It seems too informal for someone in such a high-ranking position.
"Mr" should be fine (though note that British style drops the period from common  honorifics). For example, Trinity College Cambridge's list of fellows has two "Mr"s and four "Ms"s. 

However, be very sure that the lecturer you're emailing doesn't have a doctorate. Most Oxbridge (and I believe UK) lecturers are equivalent to some sort of professor in the US system, and the title of professor is generally reserved for academics somewhere between "regular member of teaching/research staff" and "department head". Accordingly, most UK lecturers do have a PhD.

If you're not sure, it's probably better to go with "Dr" rather than "Mr", as if you do get it wrong one way or the other, that's the one that's less likely to offend.