Penn's firm was hugely significant, due to his innovations in engine and propeller systems. He was a major supplier to the Royal Navy during their transition from sail to steam power. When he took over his father's works JohnPenn & Sons was born. He shifted the firm's focus to marine engines. Advances he made to oscillating engines resulted in double the power, without increasing either the weight or space occupied. His trunk engine pairs for Royal Navy gunboats used during the Crimean War were the first mass-produced, high-pressure and high-revolution marine engines.
Penn was President of IMechE twice.