Trelawny served on multiple ships as a naval volunteer while in his teen years. He traveled to India and saw combat during engagements with the French Navy. He did not care for the naval lifestyle, however, and left at nineteen years of age without becoming a commissioned officer. After retiring from the navy, he had a brief and unhappy marriage in England. He then moved to Switzerland and later Italy where he met Shelley and Byron. He became friends with the two poets, and helped teach them about sailing. He enjoyed inventing elaborate stories about his time in the navy, and in one he claimed to have deserted and become a pirate in India. After Shelley's death, Trelawny identified his body and arranged the funeral and burial.