Obviously a huge talent, mostly associated with the Golden Age EC Comics, although he did do some select comics in the Silver Age, including the Flash Gordon series for King during their brief foray into the field in the mid-1960s.
I remember being at the Statler Hilton in 1971 for the NYC Comic Convention, and doing a couple of terrific trades in the hallways, and then (mostly) some pretty terrible trades in the hotel rooms. This one guy was trying to push off these Flash Gordon issues on me and I gave him like 25 cents for Flash Gordon #5, after he opened it and showed me the Williamson splash:
But then I made the mistake of paying 35 cents for Flash Gordon #3, which has a Williamson cover and a Ric Estrada interior.
The beauty of Flash Gordon #5, though, is well worth it. Note in particular the interesting use of the inks to create tiny patterns in the leaves and lily pads. That very distinctive look would be copied to great effect by artists like Neal Adams and Jim Aparo in the near future, but in 1967?
Here's a climactic page from that same issue: