Both companies reported Monday that participating shareholders overwhelmingly approved the deal. Westar had a special meeting in Topeka and Great Plains had one in Kansas City, Missouri, to finish the voting.
Each firm said the holders of more than 60 percent of their shares participated, and the owners of 95 percent or more of that stock approved the sale.
Federal and state regulators also must sign off. The companies hope to complete the sale next spring.
The companies have said combining the two would create efficiencies and keep consumers' rates in check. Westar stockholders would receive $51 in cash and $9 in Great Plains stock for each share.