he OED’s earliest listed usage of “log in” in the modern sense of “to open one’s on-line access to a computer” is from the 1963 publication Compatible Time-Sharing System from the MIT Computation Center. [2] I’m not sure if this is truly the first usage of “log in”, but it would make sense if it was, as CTSS, started in 1961, was arguably the first time-sharing operating systems, and so possibly the first system that you needed to log in to. (Before that we only had batch processing systems).