Malcolm Coulthard is best known for his work on the analysis of Spoken and Written Discourse and his An Introduction to Discourse Analysis (1977/1985) is still widely used.

Recently he has become increasingly involved in the area of Forensic Linguistics. He is the founding editor of Forensic Linguistics: the International Journal of Language and the Law and was the Founding President of the International Association of Forensic Linguists. He has been commissioned to write reports as an expert witness in over 150 cases and has given evidence in three terrorist trials in Northern Ireland and in a case of academic plagiarism in Hong Kong. He has appeared as an expert twice in the Court of Appeal, including the successful Derek Bentley Appeal when, in 1998, the verdict of guilty was overturned after 46 years.