Cecilia May Gibbs was born in Kent, in the United Kingdom, to Herbert William Gibbs and Cecilia Rogers, who were both talented people. She was their second child, and as she was named after her mother, had the nickname "Mamie". The family moved to South Australia to set up a farm in 1879 due to Herbert's failing eyesight, the result of a boyhood injury. However, as May had caught the measles, her father and uncle went to Australia, leaving her mother in England to care for the children. On 1 June 1881, the Gibbs brothers arrived in South Australia, and began to look for the land arranged for them by a relative of theirs. Over the next few months, the brothers became disillusioned with the land. Cecilia discovered that she was pregnant again, and decided to make the voyage to Australia with her children. Despite her parents' dismay, Cecilia and the children left, and her third child, Ivan, was born at sea. A drought in the area caused the family to move again, to Norwood. In 1885, the family moved again to a farm property in Harvey, Western Australia. When May was eight years old, she was given a pony by her father.May enjoyed exploring the bush riding her pony, Brownie,and began to paint and write about the bush at this time. This period of her childhood, and her imaginative interpretation of the bush, was formative in the development of the anthropmorphic bush setting found in her work. When May was 10, the family moved to Perth, and in 1889 May was published for the first time - in the Christmas edition of the W.A. Bulletin. A number of return trips to England found her absent from that state, but in 1905 May was working for the Western Mail. After finishing school, Gibbs spent seven years studying art in the UK. While overseas, she published her first book, About Us. In 1913 she returned to Australia, and took up residence in Neutral Bay, in Sydney, New South Wales.

1913 also marked the first public appearance of the gumnut babies, on the front cover of The Missing Button, by Ethel Turner, which Gibbs had illustrated. Gibbs' first book about the gumnut babies, appropriately titled Gumnut Babies, was published in 1916. It was soon followed, in 1918, by her most famous work, Snugglepot and Cuddlepie. Gibbs wrote many books on the theme of the gumnut babies. In addition to her work illustrating and writing, Gibbs also maintained two comic strips, Bib and Bub 1924-1967 and Tiggy Touchwood 1925-1931. The comic strips were published in newspapers in most Australian states and also in New Zealand.

Gibbs married Bertram James Ossoli Kelly, a mining agent, who she met in 1919 during a visit to Perth.

May Gibbs died in 1969, but her legacy to children lives on. Gibbs bequeathed the copyright from the designs of her bush characters and her stories to Northcott Disability Services (formerly The NSW Society for Crippled Children) and The Spastic Centre of NSW. The residue of her estate was left to the United Nations International Children's Emergency Fund.