Stevenson describes him as "a tall man, over six feet high, and broad in proportion, and he had a bluff, rough-and-ready face, all roughened and reddened and lined in his long travels.

He is somewhat gullible, being persuaded by the cunning Long John Silver into hiring a crew of mutinous pirates, and argues with Captain Alexander Smollett about the latter's "unmanly, unsailorly, and downright un-English" doubts about the safety of the cruise.

However, when Jim discovers the planned mutiny, Trelawney humbly admits his error and proves a loyal and capable companion, and is grieved when his gamekeeper, Tom Redruth, dies from a gunshot wound.

Going to the Bristol docks, Trelawney buys the schooner Hispaniola, hires Captain Smollett to command her, and retains Long John Silver, a former sea cook and now the owner of the dock-side "Spy-Glass" tavern, to run the galley.

Although the squire is financier of the expedition and appears to be the social better of all others aboard the ship, he is privately dismissed on several occasions due to his lack of discretion.