Mr. Allen is the squire of Fullerton in Wiltshire. He is a sensible, intelligent man, though he did marry Mrs. Allen, and very rich, though not as rich as General Tilney and his property is not as extensive. Because they have no children and his estate is not entailed, he is free to leave Fullerton to whomever he chooses.
He enjoys history and perhaps also science, since he mentions his barometer at home when asked about the weather. James Morland says there isn't a better judge of beauty than Mr. Allen. He has described the villains in Catherine Morland's novels as "unnatural and overdrawn." He takes a walk every day, but he never goes into the hothouse where his wife keeps her plants in the winter. Mrs. Allen complains that he does not like to wear his greatcoat, even when it rains.
Mr. and Mrs. Allen are close with their neighbors the Morlands, and so they invite Catherine Morland to join them in Bath when Mr. Allen is ordered there because of his gout. The Allens are mediocre chaperones, however. Mr. Allen retreats to the card room as soon as they enter the Upper Rooms and does not see his wife or their friend again until the ball is over. He does inquire into Henry Tilney's circumstances after he is introduced to Catherine, but he fails to check their acquaintance with the Thorpes, even after Catherine repeatedly drives out with John Thorpe and it is even proposed that they visit an inn together (with her brother and his sister); he only suggests she stop driving out with him in an open carriage.
Mr. Allen grows sick of Bath, but after six weeks, he decides they will stay another fortnight because his wife enjoys it and she thinks his health will improve further if they stay longer. At the same time Catherine is invited to visit Northanger Abbey with the Tilneys, and Mr. Allen escorts her to their lodgings in Bath on the morning they depart.
He expresses his resentment when Catherine is sent back to Fullerton by General Tilney. When Henry comes to explain what happened and propose to Catherine, he does so during a walk to visit Mr. and Mrs. Allen. General Tilney becomes reconciled to the engagement in time, in part because he thinks Catherine could still be named Mr. Allen's heir.