I've written a bit about existentialism. In all my reading on the subject (ample for a layman, little for a scholar), I've never seen William James referred to as an existentialist thinker. But that's how Jacques Barzun refers to him in this passage from his excellent biography:
[Such comments] mark him out as what came in the twentieth century to be known as an existential thinker: that is, one who philosophizes from the need to survive intellectually and emotionally in a universe that the collapse of traditional religion and the tyranny of science have laid waste.
It has always struck me that existentialism was the last big philosophical school. It started with Kierkegaard and reached a crescendo in the 1960s. After that, it kind of died away, but it never went away and was never replaced with anything. "Existentialist" still has a cutting edge sound to it that makes a kid look cool in the coffee shops. There are reasons for this, two of which are touched upon in the Barzun quote, but such an exploration defies the size of a conventional blog post.
