In his early thirties he won the design competition for the botanical gardens in Sheffield, with a plan praised by the great gardening journalist John Claudius Loudon as showing ‘a mind deeply imbued with knowledge of his profession’ (Gardeners’ Magazine, 1834, p276). Marnock moved to Sheffield to oversee the creation of the gardens and become their first curator, and was soon advising on planting and design elsewhere, including the adjacent cemetery. During this time, Marnock married Anne Hobson, the daughter of a dissenting minister and advocate of the Lancasterian system of education (which enabled the poor to be taught). This family influence greatly affected Marnock’s future clientele and the social emphasis in his designs.

In 1839 Marnock and Anne moved to Hackney (then a centre of horticulture), where he intended to settle as a nurseryman. His reputation soon brought him the commission to design a garden for the Royal Botanic Society in the inner circle of Regent’s Park. He subsequently served as curator for more than 20 years and his enormous Flower Shows became one of the horticultural events of the year.