Henry was not eligible to run for election to the Legislative Assembly in the 1965 Cook Islands election because he had not been a resident of the Cook Islands for three years. The CIP had Henry's sister, Marguerite Story, run in the Te-au-o-Tonga riding as a "stand-in" for Henry. After the CIP formed the government, it quickly changed the residency requirement from three years to three months, and Story resigned the seat so that Henry could run in the by-election. Henry won the by-election in Te-au-o-Tonga and shortly afterwards he was selected as the first Premier of the Cook Islands. (In return, Henry ensured that Story was elected Speaker of the Legislative Assembly.) Henry was re-elected consistently in the election of 1968, 1972, 1974 and March 1978. Upon being elected Premier, Henry did much to unify the Cook Islands and to promote its newly awarded self-government. His government quickly approved the proposed constitution, which awarded self-rule to the Cook Islands while maintaining New Zealand citizenship for its residents.