The Heart Is A Lonely Hunter (his first film score, I think?)
According to online sources there are several before it already, most noable of course the score to THE GRADUATE. Whihc of course is overshadowed by the Simon & Garfunkel songs.
There is a even direct connection with Williams and Grusin: Dave Grusin performed on several tracks – not only the two credited directly with Dave Grusin Trio – on Williams’ THE LONG GOODBYE from 1973.