This is how I divide his career.
The 50s represents a student period, so I would keep that separate
From about 1960-1974 we have the Early Period
Here, Williams really began his career in the concert hall and for the screen. At first, he kind of divided the two fields, trying to write "serious" concert works and lighter film and TV works. As his concert career did not really take off, Williams seemed to begin to devote more creative energies to his dramatic works, now also using the name "John" for them. Jane Eyre perhaps represents his first fully mature work.
The next period I call the Heroic Period which lasted from 1975 to about 1986
The death of his first wife, Barbara, is the event that divides the periods and, as Williams has said, marks a major turning point in his life and art. Williams's music in this period is marked by two things: subject material that reflects heroic themes and events, and characters; and, music that represents a struggle with and triumph over challenges in life. The transition piece here is the Violin Concerto, which, like his trio of disaster scores in a sense, deal with the emotions of a sudden calamity. After this, Williams seems to have become more personal in his music, even as he began scoring more and more high profile works. The intense forces of nature and exhausted triumph in Jaws. The reaching for something higher in the artisic triumph that is CE3K. The straightforward, ambitious heroics of the Star Wars movies, Superman, and Raiders of the Lost Ark, with their love themes that range from yearning, to acceptance (Death and Transfiguration quote in Superman) to romantic renewal. And, finally, the utter triumph and joy in the last sequence of E.T., where Williams, now at the pinnacle of American music, gives a symphonic statement that calls to mind the Finales of Beethoven's Heroic period.
This period is followed by what I guess I can call the Reflective Period. This is a rather brief one, lasting from 1986 to maybe 1996.
Here we find Williams thinking about his legacy, trying to craft music that can stand alone in a sense. He returned to more concert music writing. And, his film writing underwent a change to a more subtle approach. Scores like Empire of the Sun, Born on the Fourth of July, and Schindler's List are prime examples, but even his blockbuster scores, like The Last Crusade, Hook, and Jurassic Park are marked by this shift. For me, this represented Williams at his compositional peak.
Next comes what I call John's Experimental Period, which started in about 1996 and lasted until 2005 or so. (KOTCS, and the late 00s concerti represent kindsof inter-period works)
This period was marked by Williams beginning to be more eclectic in how he approached film projects. He began to be a little more adventurous with many scores, even more operatic, even, than he had been before. The Prequel Scores, A.I., and Memoirs of a Geisha are some prime examples. His concert works also began to really push the boundaries of modern romanticism.
Finally, we have John's Late Period.
Starting in about 2011, Williams started to write music that has felt a little reflective, where he has streamlined things, but has returned to a sort of straightforward vitality. Yet, the music he has written for film in this period are quite distinct from anything he has done before, even in recent years. Semi-retirement has allowed him to be more selective with his projects, and spend possibly some more time on them. Also, I find he has made strides in terms of structure in his concert works.