It follows the departure of Angie Ridgwell (LGC - 20/11/08) , the second chief executive to quit within two years.
Mr Rowan said: “My first priority is to restore trust. This will be achieved through honesty and openness, underpinned by clear governance.”
Leader Terry Hipsey (Con) said Mr Rowan had “carried out exemplary work and he will bring a balance to our corporate management team”.
Mr Rowan was previously head of legal services at Stroud and Kennet DCs.