Returning to Tucson, Arizona in 1969 to specialize in international law, Castro continued to rise to the top in Arizona Democratic politics. Seeking state office for the first time in 1970, he won the Democratic nomination for governor, but narrowly lost the general election to Republican incumbent Jack Williams. In 1974, Castro was elected governor. In 1977, after two years as governor, he was selected by President Jimmy Carter to be ambassador to Argentina. Castro held that post until 1980.