He worked in various positions throughout the series in a data processing company. He was initially a temp in data processing for five years and later promoted to data processing supervisor. This is shown in S01 E15 The One with the Stoned Guy
From Wikipedia article of Chandler Bing,
Chandler works as an IT procurements manager with the specialization Statistical analysis and data reconfiguration, which he takes up as temporary work and, despite working in the industry for years, thoroughly loathes. It is seen in many exterior shots that he works at the Solow Building.
In S08 E21 The One With the Cooking Class, He gets ready for a job interview. He mentions the name of his job.
Joey: Sure, I can hang out ‘til I have to meet ya. (To Chandler) What uh—How come you’re not going?
Chandler: I have a job interview I have to get ready for.
Joey: I thought you already have a job.
Chandler: And people say you don’t pay attention. No, this is a much better job. It’s vice-president of a company that does data reconfiguration and statistical factoring for other companies.
In S09 E10 The One with Christmas at Tulsa, he quits the job.
Monica: What're you doing here?
Chandler: I wanted to be with you. I missed you so much.
Joey: Hey, hey, uh, who did you miss the most?
Joey: Gotcha. (blinks an eye)
Chandler: I never wanna leave you again!
Monica: But I thought if you left, you get fired.
Chandler: Turns out they can't fire me. Because I quit.
After quitting the job, Monica helps Chandler secure a job in advertising. Quoting from the same Wikipedia article
Monica helps Chandler secure a job in advertising through an old colleague friend of hers. Much to Chandler's dismay, he begins as an intern, which leaves him in the awkward position of working alongside people who are significantly younger than he is. However, his more mature approach eventually pays off, and he secures a full-time job in the business as a junior copywriter, even though he only expected to receive an assistant position.
So, he worked in a data processing company for a long time as a IT procurements manager and later he worked in an advertising company as a Junior Advertising copywriter.