We first learn of Sir William Thornhill when he is disguised as Mr. Burchell, a poor and somewhat odd man. He saves Sophy (Sophia) from drowning and spends a good deal of time with her. They seem to get along well, but although Mrs. Primrose likes Mr. Burchell very much, neither she nor the vicar encourage the relationship, because of Burchell's lack of money.

Sir William's main function emerges near the end of the novel. He acts a deus ex machina or the "godlike machine" who solves many of the Primrose's problems. For example, still disguised as Mr. Burchell, he saves Sophia from her abductors, bravely chasing them off. The vicar is so grateful for this that he says he wishes Burchell would marry Sophia, despite his poverty, stating his character is worth more than thousands in money.

When it becomes clear that Mr. Burchell is in reality the wealthy, powerful, and honored Sir William Thornhill, Mrs. Primrose feels ashamed, but he reassures her not to worry. He helps apprehend his evil nephew, Squire Thornhill, who cruelly had the vicar thrown in jail for not being able to pay his rent after his house burned down. When the squire is exposed as having tried to stage a fake marriage with Olivia, Sir William realizes what a "viper" he has been harboring in his nephew and metes out punishment, limiting him to a small allowance. Finally, Sir William marries Sophia, as the two have long loved each other.

Sir William is a character of great goodness and benevolence who loves to do good and who rights many of the wrongs the innocent Primroses have suffered.