Lord John does not appear in person in The Fiery Cross, but he and Jamie exchange several letters throughout the novel. Jamie writes to John and tells him that Brianna's son is named Jeremiah Alexander Ian Fraser MacKenzie, "Ian" being the Scottish version of "John", and informs him of his task of forming a militia and leading them to do the governor's bidding. Jamie asks John to look after his family, should anything happen to him.
Later, Brianna finds a letter from Lord John in her father's study, but not the one that the family had all shared earlier. She reads that John has been helping Jamie find out about Stephen Bonnet's whereabouts, at Jamie's request. He has also been on the lookout for an astrolabe for Jamie, but one of these does not appear until some time later, and not from Lord John, but his stepson, William.