One element of the novel that has been a subject of heavy criticism is the apparent shift in genres between the political focus of the early chapters to the domestic in the later ones. Raymond Williams particularly saw this as a failure by the author: the early chapters, he said, are the 'most moving response in literature to the industrial suffering of the 1840s', but in the later the novel becomes a 'familiar and orthodox...Victorian novel of sentiment'. Williams suggested that this shift may have been at the influence of her publishers, an idea supported by the title change, which changes the main focus of the reader from the political upheaval John is trying to promote to Mary's emotional journey.
However, Kamilla Elliot disagrees with Williams about the weakness of the domestic genre, saying, 'It is the romance plot, not the political plot, that contains the more radical political critique in the novel.'
It is a subject of some debate whether the first person narrator in Mary Barton is synonymous with Gaskell. On the one hand, the consistent use of tone through the original preface and the novel, and authorial insets like the first paragraph of chapter 5 suggest the Gaskell is directly narrating the story. Contrarily, critics like Lansbury suggest the narrator is too unsympathetic in all Gaskell's Manchester novels to be her own voice:
Nothing could be more unwise than to regard the authorial 'I' of the novels as the voice of Elizabeth Gaskell, particularly in the Manchester novels. The narrator has a tendency to engage in false pleading and specious argument, while the workers demonstrate honesty and commonsense.
Hopkins goes so far as to claim that the detail to verisimilitude in the novel made it the first 'respectable' social novel, in contrast with the lack of believability in, for example, Disraeli's Sybil or Tonna's Helen Fleetwood.
Prominent in the novel is Gaskell's attempt to reinforce the realism of her representation through the inclusion of 'working-class discourses', not only through the use of closely imitated colloquialisms and dialect, but also through 'passages from Chartist poems, working class ballads, proverbs, maxims and nursery rhymes, as John Barton's radical discourse, Ben Davenport's deathbed curses, and Job Legh's language of Christian submission.'
The first half of the novel focuses mainly on the comparison between the rich and poor. In a series of set pieces across the opening chapters we are shown the lifestyles of the Bartons, Wilsons (most prominently in the chapter "A Manchester Tea-Party") and Davenports respective households compared to the contrasting affluence of the Carson establishment (in the chapter "Poverty and Death"). A key symbol shown in this chapter is the use of five shillings; this amount being the price John Barton receives for pawning most of his possessions, but also the loose change in Harry Carson's pocket.
Gaskell details the importance of the mother in a family; as is seen from the visible decline in John Barton's physical and moral well-being after his wife's death. This view is also symbolised by Job Leigh's inability to care for Margaret as a baby in the chapter "Barton's London Experience". The theme of motherhood is connected to declining masculinity: Surridge points out that the roles of nurturing fall towards the men as bread-winning falls away. Both Wilson and Barton are pictured holding the infants in the place of the nanny that can't be afforded as the novel begins, but eventually both end up relying on the income of their children, Jem and Mary respectively.
The second half of the book deals mainly with the murder plot. Here it can be seen that redemption is also a key aspect of the novel; not least because of the eventual outcome of the relationship between Messrs Carson and Barton, but also in Gaskell's presentation of Esther, the typical "fallen woman". The selfless nature she gives the character, on several occasions having her confess her faults with a brutal honesty, is an attempt to make the reader sympathise with the character of a prostitute, unusual for the time.
Indeed, throughout the novel Gaskell appears to refer to her characters as being out of her control, acting as not so much a narrator but a guide for the observing reader. Another aspect of the passivity of the characters is, as some suggest, that they a represent the impotence of the class to defend, or even represent, themselves politically. Cooney draws attention to this in the scene in which the factory is on fire – a scene the reader anticipates to be domestic fails in its domestic role (one might imagine Jem's heroism to prompt Mary to discover her true feelings) actually sees the crowd passively at the mercy of ill-equipped firemen and unconcerned masters.
Several times Gaskell attempts to mask her strong beliefs in the novel by disclaiming her knowledge of such matters as economics and politics, but the powerful language she gives to her characters, especially John Barton in the opening chapter, is a clear indication of the author's interest in the class divide. She openly pleads for reducing this divide through increased communication and, as a consequence, understanding between employers and workmen and generally through a more human behaviour based on Christian principles, at the same time presenting her own fears of how the poor will eventually act in retaliation to their oppression.
Gaskell also describes an Italian torture chamber where the victim is afforded many luxuries at first but in the end the walls of the cell start closing in and finally they crush him. It is believed that the story has been influenced by William Mudford's short story "The Iron Shroud". Stephen Derry mentions that Gaskell uses the concept of the shrinking cell to describe John Barton's state of mind but also added the element of luxury to further enhance it.
Death plays a significant and unavoidable role in the plot: it has been interpreted both as mere realism (Lucas points out the average mortality rate at the time was 17) and autobiographically as the cathartic relief of grief over her son's premature death. The image of a dying child was also a trope of Chartist discourse.