He studied civil engineering and worked with the famous engineer Isambard Kingdom Brunel. Between 1850 and 1860, Trewhella was invited by the Italian government to participate in the construction of the infrastructure of the country. He moved to Italy, designing and building railways, roads and bridges, including the seventy-mile line between Florence and Bologna through the rugged Apennine Mountains. He built various railways in Sicily, including the Circumetnea line around Mount Etna, and the Palermo-Corleone line. He acquired land and sulphur mines, and built the first great hotel in Palermo, the Excelsior, where, in 1903 he received King Edward VII and Queen Alexandra as his guests. He is associated in some manner with the Palazzo Trewhella, 91-103 Via Garibaldi, on the south side of that street, west of the junction with Via Santa Chiara, Catania, a large 18th century apartment block surrounding a central courtyard.