Mikkel studied History of Art at the University of Copenhagen, Denmark, 1973-1977. He obtained a B.Sc. in Painting Conservation 1980 and an M.Sc. in Painting Conservation 1987 from the School of Conservation at the Royal Danish Academy of Fine Arts, Copenhagen, Denmark. 1980-1987 he worked and lectured as conservator in Denmark and abroad. From 1987 lecturer at the School of Conservation, from 1992 Head of Department of Paintings and from 2004 Head of the Department of Monumental art as well.

Mikkel has a variety of lecturing and/or research interests, specializing in structural treatment of canvas paintings (e.g., suction table techniques), treatment of contemporary art, various kinds of analytical photography (e.g., X-ray, UVF, IRR) and preventive conservation (micro climate, art in transit). Other interests include the technique (technical art history) of early medieval painting on wood (especially coloured grounds), medieval wall paintings, polychromy of antique Greek & Roman sculpture and the history of conservation of paintings. As well as lecturing, conducts research and has written various publications in these fields.

Previously coordinator, later assistant coordinator, for the ICOM Committee for Conservation's (ICOM-CC) Working Group on Painting Conservation, Treasurer on the ICOM-CC Board and afterwards co-opted to the ICOM-CC Board for a third term. Member of the Board of the Danish National ICOM committee, among other things. At present elected Council member, International Institute for Conservation (IIC). Mikkel is one of the founding members of INCCA.