Mr. Mell is one of Salem House's three masters, along with Mr. Sharp and Mr. Creakle. He is a shabby, down-at-heel man who comes to meet David at a London inn when David first travels to his new boarding school. Mr. Mell is kindly, and thus entirely out of place at a dive like Salem House. But Mr. Mell also has a secret: his mother is at a poor house, a shelter for people with absolutely no money. In a society so completely determined by class like the one in this novel, it is social suicide that Mr. Mell comes from such poor relations.
David meets Mr. Mell's mother and is impressed by her pride in her son's accomplishments. But David is naive, and makes the mistake of telling all of his school friends, including Steerforth, about Mrs. Mell's extreme poverty. Even as he tells them, he feels guilty about betraying Mr. Mell's secret. But David can't predict the end result of his indiscretion.
One day, Mr. Mell is alone in the classroom because Mr. Creakle, the headmaster, is feeling sick. The boys use this as an excuse to get up to all kinds of tricks. They also start mocking Mr. Mell to his face. Mr. Mell scolds them for behaving so badly to a gentleman. Steerforth takes offense at Mr. Mell's words, and retorts that he won't take any kind of discipline from a man like Mr. Mell. Mr. Creakle comes into the classroom to investigate all of this noise, and finds that Mr. Mell is trying to punish Steerforth (who is Mr. Creakle's favorite student). Steerforth tells Mr. Creakle that Mr. Mell's mother is in the poor house, and Mr. Creakle fires Mr. Mell on the spot.
It is this cruel treatment of poor Mr. Mell that first tips off David that maybe Steerforth isn't such a great guy after all. Unexpectedly, it is Tommy Traddles who stands up against Steerforth, protesting that he has treated Mr. Mell very badly. So it is in an argument over Mr. Mell that we first see the strong contrast between David's two main influences from school, Traddles and Steerforth. Steerforth smoothes over David's concerns by promising that he is going to write to his mother to get Mr. Mell a job (...right). But David never forgets Steerforth's sneering at Mr. Mell's mother.
Even though he's a minor character, we get this totally random news that Mr. Mell winds up OK. Dickens really hates loose ends! When Mr. Peggotty delivers a newspaper to David with news from his new home in Australia, David finds a reference to "Doctor Mell," the headmaster of a successful school called Salem House. So, that's karma for you – Mr. Mell moved to Australia and started a school using the name of the place he was fired from all those years ago. But he's doing well and is respected in the British colonies. Random, right?