Dr Guest started out at Empress Nickel Mine in Rhodesia progressing to Resident Geologist in charge of both the open pit and underground operations. Empress Nickel Mine was a nickel-copper operation, using blasthole open stoping.
In 1977, he moved to South Africa, where the first two years were spent working in the deep level gold mines, both at Vaal Reefs Gold Mine in the Klerksdorp area and for the Chamber of Mines Research Organisation, in Johannesburg.
In 1979, he moved to Kimberley, South Africa to work with De Beers Consolidated Mines Limited. Whilst in Kimberley, he became involved in all of the De Beers underground operations as well as the open pit operations in Botswana.
In 1990, he moved back to Johannesburg, Head Office, with responsible for Geotechnical and Blasting aspects at all De Beers operations in Southern Africa, Russia and Canada. He also become involved with all Anglo American's South American operations.
In 2007, he retired from De Beers Group after 29 years. Presently Dr. Guest is consulting to the mining industry worldwide.