Loudon went to school in Edinburgh and then served as apprentice to a nurseryman and landscape gardener at Easter Dalry, Scot. He moved to London in 1803 and quickly established himself as a successful landscape gardener; in the same year he wrote a notable essay on the laying out of public squares. His prodigious publishing activity began in 1806 with his first book on gardening, which was followed by numerous others covering all aspects of horticulture, landscape design, and related subjects. Among his publications were An Encyclopaedia of Gardening (1822), the successful monthly, Gardener's Magazine, and a major work, Arboretum et Fruticetum Britannicum (1838).

Loudon advocated irregular, picturesque gardens that were simultaneously intended as settings for botanical study. He called his style the “Gardenesque,” in contrast to the more visually and artistically oriented Picturesque. With its rather moralistic aim of combining instruction and pleasure, Loudon's style became the dominant influence on Victorian taste in gardens. The epitome of his approach is the concept of the arboretum—a place where trees and shrubs are cultivated for the purpose of observation and study—exemplified by his most important work, the Derby Arboretum (1839–41).

Loudon's involvement with architecture arose naturally out of his interest in landscape. He made himself a specialist in rural vernacular building types by writing his Encyclopaedia of Cottage, Farm, and Villa Architecture (1833). This work was unprecedented in that it was consciously addressed to the middle class rather than to an aristocratic audience. It thus helped shape Victorian suburban architecture.