Internationally he is acknowledged as the co-inventor of object oriented programming and the programming language Simula with Ole - Johan Dahl in the 1960s. Object - oriented programming enables software developers to manage the complexity of computer systems.

Nygaard got his master's degree in mathematics at the University of Oslo in 1956. His thesis on abstract probability theory was entitled "Theoretical Aspects of Monte Carlo Methods".

Nygaard worked full time at the Norwegian Defense Research Establishment from 1948 to 1960 - in computing and programming (1948 – 1954) and operations research (1952 – 1960).

From 1957 to 1960 he was head of the first operations research group in the Norwegian defense establishment. He was cofounder and first chairman of the Norwegian Operational Research Society (1959 – 1964). In 1960 he was hired by the Norwegian Computing Center (NCC), responsible for building up the NCC as a research institute in the 1960s, becoming its Director of Research in 1962.

Together with Ole - Johan Dahl he developed SIMULA I (1961 – 1965) and SIMULA-67 - the first object - oriented programming languages, introducing the concepts upon which all later object - oriented programming languages are built: objects, classes, inheritance, virtual quantities and multi - threaded (quasi - parallel) program execution. In 2004, AITO established an annual prize in the name of Ole - Johan Dahl and Kristen Nygaard to honor their pioneering work on object - orientation. The AITO Dahl - Nygaard prize is awarded annually to two individuals that have made significant technical contributions to the field of Object - Orientation. The work should be in the spirit of the pioneer conceptual and/or implementation work of Dahl and Nygaard which shaped our present view of object - oriented programming. The prize is presented each year at the ECOOP conference. The prize consists of two awards given to a senior and to a junior professional.

He did research for Norwegian trade unions on planning, control, and data processing, all evaluated in light of the objectives of organised labour (1971 – 1973), working together with Olav Terje Bergo. His other research and development work included the social impact of computer technology and the general system description language DELTA (1973 – 1975), working with Erik Holbaek - Hanssen and Petter Haandlykken.

Nygaard was a professor in Aarhus, Denmark (1975 – 1976) and then became professor emeritus in Oslo (part - time from 1977, full time 1984 – 1996). His work in Aarhus and Oslo included research and education in system development and the social impact of computer technology, and became the foundation of the Scandinavian School in System Development, which is closely linked to the field of participatory design.

In June 1990, he received an honorary doctorate from Lund University, Sweden, and in June 1991 he became the first person to be given an honorary doctorate by Aalborg University, Denmark. He became a member of the Norwegian Academy of Sciences.

In October 1990, Computer Professionals for Social Responsibility awarded him its Norbert Wiener Award for Social and Professional Responsibility.

In 1999, he became — together with Dahl — the first to receive the Rosing Prize. This new prize is awarded by the Norwegian Data Association for exceptional professional achievements.

In June 2000, he was awarded an Honorary Fellowship for "his originating of object technology concepts" by the Object Management Group, the International Organization for Standardization within object - orientation.

In November 2001, he and Dahl were awarded the IEEE John von Neumann Medal by the Institute of Electrical and Electronic Engineers "For the introduction of the concepts underlying object - oriented programming through the design and implementation of SIMULA 67".

In February 2002, he was given, once more together with Ole - Johan Dahl, the 2001 A.M. Turing Award by the Association for Computing Machinery (ACM), with the citation: "For ideas fundamental to the emergence of object oriented programming, through their design of the programming languages Simula I and Simula 67."

In August 2000, he was made Commander of the Royal Norwegian Order of St. Olav by the King of Norway.

Beginning in 1976, he was engaged in the development and (since 1986) the implementation of the general object - oriented programming language BETA (together with Bent Bruun Kristensen, Ole Lehrmann Madsen and Birger Moeller - Pedersen). The language is now available on a wide range of computers.

Nygaard was in the first half of the 1980s chairman of the steering committee of the Scandinavian research program SYDPOL (System Development and Profession Oriented Languages), coordinating research and supporting working groups in system development, language research and artificial intelligence. Also in the 1980s, he was chairman of the steering committee for the Cost-13 (European Common Market Commission) - financed research project on the extensions of profession - oriented languages necessary when artificial intelligence and information technology are becoming part of professional work.

Nygaard's research from 1995 - 1999 was related to distributed systems. He was the leader of General Object - Oriented Distributed Systems (GOODS), a three - year Norwegian Research Council - supported project starting in 1997, aiming at enriching object - oriented languages and system development methods by new basic concepts that make it possible to describe the relation between layered and/or distributed programs and the hardware and people carrying out these programs.

The GOODS team also included Haakon Bryhni, Dag Sjøberg and Ole Smørdal.

Nygaard's final research interests were studies of the introductory teaching of programming, and the creation of a process - oriented conceptual platform for informatics. These subjects are to be developed in a new research project called COOL (Comprehensive Object - Oriented Learning) together with a number of international test sites. He was giving lectures and courses on these subjects in Norway and elsewhere. In November 1999 he became chair of an advisory committee on Broadband Communication for the Norwegian Department for Municipal and Regional Affairs. He held a part time position at Simula Research Laboratory from 2001, when the research institute was opened.

In 1984 and 1985 Nygaard was chairman of the Informatics Committee of the University of Oslo, and active in the design of the university's plan for developing research, education and computing and communication facilities at all faculties of the university.

He was the first chairman of the Environment Protection Committee of the Norwegian Association for the Protection of Nature.

He was for 10 years (in the 1970s) Norwegian representative in the OECD activities on information technology. He has been a member of the Research Committee of the Norwegian Federation of Trade Unions, and cooperated with unions in a number of countries.

He was for several years engaged in running an experimental social institution trying new ways of creating humane living conditions for socially outcast alcoholics.

Nygaard was active in Norwegian politics. In the mid and late 1960s he was a member of the National Executive Committee of the Norwegian liberal party Venstre, and chair of that party's Strategy Committee. During the intense political fight before the 1972 referendum on whether Norway should become a member of the European Common Market (later the European Union), he worked as coordinator for the many youth organisations that worked against membership.

From 1971 to 2001 Nygaard was a member of the Labour Party, and he was a member of committees on research policies in that party.

In November 1988, he became chair of the Information Committee on Norway and the EEC, in August 1990 reorganized as Nei til EF an organization disseminating information about Norway's relation to the Common Market, and coordinating the efforts to keep Norway outside. (No to European Union membership for Norway, literally "No to the EU"). In 1993 when the EEC ratified the Maastricht treaty and became the European Union the organization changed its name to reflect this. Nei til EF became the largest political organization in Norway (145,000 members in 1994, from a population of 4 million). Nygaard worked with Anne Enger Lahnstein, leader of the anti - EU Centre Party, in this campaign. In the referendum on November 28, 1994, "Nei til EU" succeeded: 52.2% of the electorate voted "No", and the voter participation was the highest ever in Norway's history - 88.8%. The strategy of the campaign, insisted by Nygaard, was that it had to be for something as well as against, i.e. the Scandinavian welfare state Nygaard considered threatened by the Maastricht Agreement.

He resigned as chair in 1995, and was later the chair of the organization's strategy committee and member of its Council.

In 1996 and 1997 Nygaard was the coordinator of the efforts to establish The European Anti - Maastricht Movement (TEAM), a cooperative network between national organizations opposing the Economic and Monetary Union of the European Union (EMU) and the Maastricht Treaty in European countries within and outside the EU. TEAM was successfully started March 3, 1997.

Kristen Nygaard married Johanna Nygaard in 1951. Johanna Nygaard worked at the Norwegian Agency for Aid to Developing Countries. She specialized for a number of years in recruiting and giving administrative support to specialists working in East Africa. Johanna and Kristen Nygaard had three children and seven grandchildren.