While living in Monterey, Greene became a key figure in the development of the city. By 1894, his Monterey Electric Light and Development Company was providing electricity to much of Monterey. He organized the Monterey & Fresno Railroad and became vice-president of the Monterey & Pacific Grove Street Railroad. He also created and invested heavily in the Bank of Monterey, serving briefly as the ceremonial first cashier. Greene was 80 years old when he saw the successful culmination of his twenty-year campaign for the building of a breakwater to protect Monterey harbor. At considerable personal expense, Greene, who became known as "Breakwater Harry," had lobbied both the California State Legislature and the US Congress for the necessary funds. At a ceremony in 1932 to mark the start of the breakwater's construction, Greene pulled the lever which sent the first granite boulders into the bay.
Greene’s civic activism also extended to the preservation of Monterey's history. In 1896, he was Director General of the California Jubilee and in 1899 led the campaign by the Native Sons of the Golden West to save Colton Hall, the site of California's first constitutional convention, from demolition. Greene found more desirable land for the new school which the city planned to build on the site of Colton Hall and organized a public subscription to raise money for its purchase.
Six years later, Greene helped to rescue and preserve the remains of the historic Vizcaíno-Serra Oak. In 1903, the huge centuries-old tree was damaged by workmen who eventually chopped it down in 1905 and threw it into the Monterey Bay. With the help of local fisherman, the pastor of the Cathedral of San Carlos Borromeo had the tree fished out of the bay while Greene provided financial help to preserve the remains of the trunk and erect it behind the cathedral. He also commissioned local craftsmen to make several chairs out of its branches, two of which he donated to the local parlor of the Native Sons of the Golden West.