The spinor helicity formalism is a modern technique widely used in scattering amplitude calculations nowadays. However, it is hard to find a reference for who first came up with the formalism. Maybe someone here knows of the first author to ever use spinor helicity?
Even this review does not have any reference on the topic. However, they naturally introduce it from a discussion on Dirac and Weyl spinors, so I guess one can attribute the formalism to them.