[1] At the University of Copenhagen, Keiding successively held assistant, associate and, from 1984, full professor positions, the latter in the section of Biostatistics within the Faculty of Health Sciences.

In 1971 he was one of the founders of the Danish Society of Theoretical Statistics [dk] (DSTS), serving as its secretary from 1971–1975.

[3] With coauthors he contributed to event history analysis, including to models where only partial information is available.

[5] His book, Statistical models based on counting processes, written jointly with Per Kragh Andersen, Ørnulf Borgan, and Richard D. Gill, has been cited extensively in the medical literature[6] and been lauded for its "lucid" exposition of theoretical and practical aspects.

[7] Keiding co-authored an influential meta-analysis, known as Carlsen study after the name of the first author, that showed a significant decline in human sperm count and volume from 1938 to 1990.