After graduation, he returned with his colleague and friend Eduard Schaubert to Greece, where they were appointed public engineers by Ioannis Kapodistrias. On 1832 they created a new city plan for Athens, which included wide avenues, gardens and grand public buildings and gave the first street names in Athens. The plan was however simplified by Leo von Klenze, as it was considered too expensive. After disagreements with the administration, Kleanthis resigned his position.
Kleanthis gained great wealth not only through architecture, but also through the marble quarries he operated on the island of Paros. Marble from his quarries received a gold medal at the London Great Exhibition in 1851.
On 1862 he was seriously injured in an accident in a quarry and he was transported to Athens, where he died.[citation needed]