Born in Bronxville, New York, Ritchie graduated from Harvard University with degrees in physics and applied mathematics. In 1967, he began working at the Bell Labs Computing Sciences Research Center.

Ritchie is best known as the creator of the C programming language and a key developer of the Unix operating system, and as co-author of the definitive book on C, The C Programming Language, commonly referred to as K&R (in reference to the authors Kernighan and Ritchie).

Ritchie's invention of C and his role in the development of Unix alongside Ken Thompson have placed him as an important pioneer of modern computing. The C language is still widely used today in application and operating system development, and its influence is seen in most modern programming languages. Unix has also been influential, establishing concepts and principles that are now well-established precepts of computing.

Ritchie has said that creating the C language "looked like a good thing to do" and that anyone else in the same place at the same time would have done the same thing, though Bell Labs colleague Bjarne Stroustrup, developer of C++, said that "if Dennis had decided to spend that decade on esoteric math, Unix would have been stillborn."[citation needed]

Following the success of Unix, Ritchie continued research into operating systems and programming languages with contributions to the Plan 9 and Inferno operating systems and the Limbo programming language.[citation needed]

In 1983, Ritchie and Ken Thompson jointly received the Turing Award for their development of generic operating systems theory and specifically for the implementation of the UNIX operating system. Ritchie's Turing Award lecture was titled "Reflections on Software Research".

On April 21, 1999, Thompson and Ritchie jointly received the 1998 National Medal of Technology from President Bill Clinton for co-inventing the UNIX operating system and the C programming language which together have led to enormous advances in computer hardware, software, and networking systems and stimulated growth of an entire industry, thereby enhancing American leadership in the Information Age.

Dennis Ritchie is often referred to as "dmr" (his Bell Labs email address) in technical discussion groups.

Richard Hamming (1988) · Irving S. Reed (1989) · Dennis Ritchie / Ken Thompson (1990) · Elwyn Berlekamp (1991) · Lotfi A. Zadeh (1992) · Jorma Rissanen (1993) · Gottfried Ungerboeck (1994) · Jacob Ziv (1995) · Mark Semenovich Pinsker (1996) · Thomas M. Cover (1997) · David D. Clark (1998) · David A. Huffman (1999) · Solomon W. Golomb (2000) · Alexander G. Fraser (2001) · Peter Elias (2002) · Claude Berrou / Alain Glavieux (2003) · Jack Keil Wolf (2004) · Neil Sloane (2005) · Vladimir Levenshtein (2006) · Abraham Lempel (2007) · Sergio Verdú (2008) · Peter Franaszek (2009) · Whitfield Diffie / Martin Hellman / Ralph Merkle (2010)

Alan Perlis (1966) · Maurice Vincent Wilkes (1967) · Richard Hamming (1968) · Marvin Minsky (1969) · James H. Wilkinson (1970) · John McCarthy (1971) · Edsger W. Dijkstra (1972) · Charles Bachman (1973) · Donald Knuth (1974) · Allen Newell / Herbert Simon (1975) · Michael O. Rabin / Dana Scott (1976) · John Backus (1977) · Robert Floyd (1978) · Kenneth E. Iverson (1979) · C. A. R. Hoare (1980) · Edgar F. Codd (1981) · Stephen Cook (1982) · Ken Thompson / Dennis Ritchie (1983) · Niklaus Wirth (1984) · Richard Karp (1985) · John Hopcroft / Robert Tarjan (1986) · John Cocke (1987) · Ivan Sutherland (1988) · William Kahan (1989) · Fernando J. Corbató (1990) · Robin Milner (1991) · Butler Lampson (1992) · Juris Hartmanis / Richard Stearns (1993) · Edward Feigenbaum / Raj Reddy (1994) · Manuel Blum (1995) · Amir Pnueli (1996) · Douglas Engelbart (1997) · Jim Gray (1998) · Fred Brooks (1999) · Andrew Yao (2000) · Ole-Johan Dahl / Kristen Nygaard (2001) · Ron Rivest / Adi Shamir / Leonard Adleman (2002) · Alan Kay (2003) · Vint Cerf / Bob Kahn (2004) · Peter Naur (2005) · Frances E. Allen (2006) · Edmund M. Clarke / E. Allen Emerson / Joseph Sifakis (2007) · Barbara Liskov (2008) · Charles P. Thacker (2009)