 Before he came to Sheffield, Marnock worked as the head gardener in Bretton Hall (now the Yorkshire Sculpture Park), Wakefield between 1829 and 1833.He was appointed by the Sheffield Botanical and Horticultural Society in 1833 to design and lay out the Botanical Gardens, at an annual salary of £100.Marnock designed the Botanical Gardens in the then highly fashionable Gardenesque style.