[3] After his graduation, he left for Zürich and soon undertook research for Hermann Weyl and Rudolf Fueter and was awarded a doctorate in 1921 for his thesis on the tensor calculus and linear groups of matrices.

[3][4] Weyl suggested Weinstein to several prominent mathematicians, including Paul Sophus Epstein, who was then worked at the California Institute of Technology.

[1][3] With Levi-Civita, Weinstein published three more works before he returned to Zürich as a privatdocent in Weyl's chair, then in 1928 he was appointed to the Hamburg Technical University, he also joined the German Mathematical Society.

[2] By 1933, he was sought by Albert Einstein as a collaborator in Berlin, however after the electoral success of the Nazi Party, Weinstein, being of Jewish background, instead went to Sorbonne and the Collège de France in Paris, where he worked with Jacques Hadamard.

[2][4] Weinstein's method was later developed to give accurate bounds of eigenvalues of plates and membranes, and he introduced a new branch of potential theory through his examination of singular partial differential equations.