Ritchie is best known as the creator of the C programming language, one of the developers of the Unix operating system, and co-author of the book The C Programming Language; he was the 'R' in K&R (a common reference to the book's authors Kernighan and Ritchie). Ritchie worked together with Ken Thompson, who is credited with writing the original version of Unix; one of Ritchie's most important contributions to Unix was its porting to different machines and platforms.[17] They were so influential on Research Unix that Doug McIlroy later wrote, "The names of Ritchie and Thompson may safely be assumed to be attached to almost everything not otherwise attributed."[18]

Ritchie liked to emphasize that he was just one member of a group. He suggested that many of the improvements he introduced simply "looked like a good thing to do", and that anyone else in the same place at the same time might have done the same thing.

Nowadays, the C language is widely used in application, operating system, and embedded system development, and its influence is seen in most modern programming languages. C is a low level language with constructs closely translating to the hardware's instruction set. However, it is not tied to any particular hardware—making it easy to write programs on any machine that supports C.[19] Moreover, C is a high level language with constructs mapping to the application's data structures.

C influenced several other languages and derivatives as C++, Objective-C used by Apple, C# used by Microsoft, and Java extensively used in corporate environment and also by Android. Ritchie and Thompson used C to write UNIX. Unix has been influential establishing computing concepts and principles that have been widely adopted.

In an interview from 1999, Ritchie clarified that he saw Linux and BSD operating systems as a continuation of the basis of the Unix operating system, and as derivatives of Unix:[20]

I think the Linux phenomenon is quite delightful, because it draws so strongly on the basis that Unix provided. Linux seems to be among the healthiest of the direct Unix derivatives, though there are also the various BSD systems as well as the more official offerings from the workstation and mainframe manufacturers.

In the same interview, he stated that he viewed both Unix and Linux as "the continuation of ideas that were started by Ken and me and many others, many years ago.