Tom Buchanan belongs to the leisure class and doesn't have to work because he inherited a fortune. His chief interest and activity is playing polo. He owns "a string of polo ponies," something that became a stereotyped symbol of riches in novels and movies for years. When he finally condescends to attend one of Gatsby's wild parties, Gatsby takes malicious pleasure in introducing him as "Tom Buchanan, the polo player." Tom doesn't like it because it makes him sound like a playboy and a loafer. If he had more to occupy his time he probably wouldn't be getting involved in illicit affairs with women like Myrtle Wilson, whose vulgarity reflects adversely on him. He and Daisy do a lot of traveling because they are easily bored. He has taken to reading books, but Nick describes his thoughts and interests contemptuously. Tom compares unfavorably with Gatsby, who is a self-made man.
It is one of the ironies of the narrative of The Great Gatsby that little is also known of the background of Tom Buchanan and the history of his family's money just as Gatsby's source of wealth is questionable. In Chapter One, Nick Carraway narrates that Tom comes from a wealthy family from Chicago's elite suburb of Lake Forest, bringing with him to East Egg a "string of polo ponies." However, doubt is raised about Tom's pedigree with Nick's comment,
It was hard to realize that a man in my own generation was wealthy enough to do that.
It is also ironic that having come from Chicago, Tom would feel himself the equal of the other wealthy people in the East. For, Chicago has always been perceived by many Easterners as an unrefined city of the Mid-West with its stockyards and criminal element. Certainly, to this day many Easterners do not consider Chicago or its inhabitants equal to those of long-established heritage and name from New England. Certainly, Tom's ostensible display of his wealth with his polo ponies and his gift of a $350,000 pearl necklace to Daisy indicates that he is not from one of those families of generational prestige and wealth and good taste. Also, that he possesses such wealth as a man of his thirties suggests that, perhaps, his money was ill-gotten.
Tom has no formal occupation, no real "job" in the way that we conceive of a career. He played football at Yale, the same school where Nick went, and his family has a ton of money, so much money that Tom never has to work or worry about where his money will come from; Nick says that the Buchanans are "enormously wealthy."
However, despite his enormous wealth and polo ponies, Tom seems to have reached his personal peak in college and has been restlessly drifting "here and there" to wherever rich people are congregating, and Nick feels that he will continue to drift, "seeking [...] for the dramatic turbulence of some irrecoverable football game." Thus, with no reason to work now, and with nothing to define him since college, Tom is really just an ex-football player looking for the next thing he can win, the next thing that will define him. This listlessness helps to explain why Tom can actually feel put upon, a victim, and why he feels so completely concerned about the "white race" being "submerged" by the "'Colored Empires.'"
It never said that he truly had a job in the book, since we often see him basking in the light that is his money. However, during a party at one of Gatsby's parties, he was said to have been a sportsman. I believe that he had played polo. He had also gone to Yale. The book did that Tom played football during the time that he knew Nick years before. Most of his wealth did come also because of the wealth that he was seemingly born into.
Tom Buchanan is a broker and he is old money.