Lloyd was born into an upper middle class family, the youngest of six children. In 1910, his father, Nathaniel Lloyd (an Arts and Crafts designer of posters and other images for confectionary companies), purchased Great Dixter, a manor house in Northiam, East Sussex near the south coast of England. Edwin Lutyens was hired to renovate and extend the gardens attached to the house. Nathaniel Lloyd loved gardens, designed some of the garden himself, and imparted that love to his son. Lloyd learned the skills required of a gardener from his mother Daisy, who did the actual gardening and who introduced him to Gertrude Jekyll.

After Rugby School, he attended King's College, Cambridge, where he read modern languages before entering the Army during World War II. After the war he received his bachelors in Decorative Horticulture (Designing and Planning) from Wye College, University of London, in 1949. He stayed on there as an assistant lecturer in Horticulture until 1954.

In 1954, Lloyd moved home to Great Dixter and set up a nursery, specialising in unusual plants. He regularly opened the house and gardens to the public.

In 1979 Lloyd received the Victoria Medal of Honour, the highest award of the Royal Horticultural Society, for his promotion of gardening and his extensive work on their Floral Committee. Lloyd was awarded an honorary doctorate from the Open University in 1996 and was appointed as an officer of the Order of the British Empire in 2000.

Lloyd was firmly rooted in the Arts and Crafts style of garden. In most ways he was, like his mother and Gertrude Jekyll, a practical gardener. He said “I couldn’t design a garden. I just go along and carp.” Despite his extensive work with flowers, he had an appreciation for the garden as a whole. He also understood human nature. One professional gardener likes to quote Lloyd from his book Foliage Plants where he says: “For it is an indisputable fact that appreciation of foliage comes at a later stage in our education, if it comes at all”.

Lloyd rapidly felt the need to share his gardening discoveries and published The Mixed Border in 1957, which was followed by Clematis in 1965 and The Well-Tempered Garden in 1970. Lloyd had begun a book on the use of exotic plants in British gardens when he died, his gardening friends and colleagues completed the book, Exotic Planting for Adventurous Gardeners, in 2007.