There are more than 19,000 exhibits in the Harju County Museum collection, the larger collections being the photo collection, archive collection, and the collection of historical objects. For researchers, the museum also has a library.

The main part of the archaeology collection is comprised of findings from and around the small stronghold of Keila and findings from Harju-Risti Church. The more remarkable exhibits are the large copper pot found during the conservation of the Padise Monastery walls, and the chain with a pendant depicting Saint George, found from the Keila stronghold. The historical collection features ethnographic objects, tools, school furniture, garments, shoes, home appliances and much more. The archive collection contains documents, books, magazines, newspapers, ad-prints, and manuscripts. The art collection showcases graphics, drawings, paintings, and sculptures depicting various places in Harju County and/or created by artists tied to Harju County.
The photo collection has photos, negatives, and postcards mainly from the different places and areas of life in Harju County. It has consistent collections on Harju County’s community buildings, mansions, churches, and small islands, photographed in the 1990s. One of the older photos is a large-format view on the monument of Martin Luther near Keila.