Gertrude Jekyll was born at 2 Grafton Street, Mayfair, London, the fifth of the seven children of Captain Edward JH Jekyll, an officer in the Grenadier Guards, and his wife Julia Hammersley. Her younger brother, the Reverend Walter Jekyll, was a friend of Robert Louis Stevenson, who borrowed the family name for his famous novella Dr Jekyll and Mr Hyde. In 1848 her family left London and moved to Bramley House, Surrey, where she spent her formative years. She always like gardens with pretty flowers in them.

Jekyll should be more correctly categorized as a planter than as a "designer". She did indeed design, but did it through her plantings rather than traditional design aspects. She was one half of one of the most influential and historical partnerships of the Arts and Crafts movement, thanks to her association with the English architect, Sir Edwin Lutyens, for whose projects she created numerous landscapes, and who designed her home Munstead Wood, near Godalming in Surrey. (In 1900, Lutyens and Jekyll's brother Herbert designed the British Pavilion for the Paris Exposition.)

Jekyll is remembered less for her outstanding designs but instead for her subtle, painterly approach to the arrangement of the gardens she created, particularly her "hardy flower borders" (not herbaceous borders). Her work is known for its radiant colour and the brush-like strokes of her plantings; it is suggested by some that the Impressionistic-style schemes may have been due to Jekyll's deteriorating eyesight, which largely put an end to her career as a painter and watercolourist.

Jekyll was one of the first of her profession to take into account the colour, texture, and experience of gardens as the prominent authorities in her designs, and she was a life-long fan of plants of all genres. Her theory of how to design with colour was influenced by painter JMW Turner and by Impressionism. Later in life, Jekyll collected and contributed a vast array of plants solely for the purpose of preservation to numerous institutions across Britain. This pure passion for gardening was started at South Kensington School of Art,where she fell in love with the creative art of planting, and even more specifically, gardening. At the time of her death, she had designed over 400 gardens in Britain, Europe and even a few in North America. Jekyll was also known for her prolific writing. She penned over fifteen books, ranging from Wood and Garden and her most famous book Colour in the Flower Garden, to memoirs of her youth. Jekyll did not want to limit her influence to teaching the practice of gardening, but to take it a step further to the quiet study of gardening and the plants themselves.

Jekyll later returned to her childhood home in the village of Bramley, Surrey to design a garden in Snowdenham Lane called Millmead. She was also interested in traditional cottage furnishings and rural crafts, and concerned that they were disappearing. Her book Old West Surrey (1904) records many aspects of 19th century country life, with over 300 photographs taken by Jekyll.

She is buried in the churchyard of St. John the Baptist, Busbridge, Godalming, next to her brother and sister-in-law, Sir Herbert Jekyll, KCMG and Lady Agnes Jekyll, DBE. The monument was designed by Sir Edwin Lutyens.