It received highly positive reviews, praised for its intensity of natural imagery, balance of humour with difficult themes, and emotional weight.

Robinson only had experience at writing books for very young children, and was well known for her Teddy Robinson series that followed the make-believe interactions between a girl and a teddy bear, and later her Mary-Mary series that followed the mischievous family life of a young girl, which Peter Vansittart described as "slightly more sophisticated".

[1] The creation of When Marnie Was There began during a family summer holiday in the coastal village of Burnham Overy Staithe, according to Robinson's daughter.

[2] Major natural features of the location include a creek and tidal marsh, with an isolated beach surrounded by sandhills.

[3][page needed] During low tide, Robinson walked from the beach along the marsh path, watchful of a house with distinctive blue doors and window frames, named "The Granary".

[2][4] Throughout the remainder of her stay in the village, she spent time in the sandhills and wrote notes, creating the characters of Anna and Marnie.

[3][page needed] Robinson continued to write more notes after that summer, which were collated and developed into a manuscript over the course of about 18 months.

Anna finds a boat one night and rows towards the Marsh House where outside she encounters a blonde-haired girl named Marnie.

Anna explains she’s adopted, tearfully confiding that her foster parents are paid a care allowance, doubting she's sincerely loved.

Marnie admits a maid abuses her, and threatens to shut her in a nearby dilapidated windmill, and that she has an older cousin, Edward, who she alternately spends time with.

Scilla's mother, Mrs. Lindsay, interprets the archaic language and events, estimating the diary dates to First World War.

[6] Translation rights to the book were sold in 10 countries in 2016, including Japan, Italy, Spain and China, and was reprinted by HarperCollins.

Robinson wrote the novel based on her experiences of alienation and loneliness in her childhood, and said that she modelled the relationship between Anna and Marnie after her own relationship with her mother, where Robinson considered herself as Anna and her mother as Marnie, who is depicted in the story as having an elusive presence and an unreachable emotional state.

Robinson explained that writing the character of Marnie helped her forgive her mother, and accept she was "frail and human".

Vansittart described the novel, along with Robinson's later works, as exemplary in its directness of depicting the psychological profile of a girl who feels she is misunderstood, spoils everything, stores up vindictive thoughts, is resentful, expects disappointment, and endures "the dreamy poetry of growing up.

British writer Naomi Lewis commented on chapters that deal with Marnie's disappearance, writing "the salt smell, the slapping of waves, the bird that seems to cry 'Pity me!'

Chau-Yee Lo wrote that "the saltiness of its sea air, the quiet, gentle rhythm of life which enables reflection, lending a sense of coming to the farthest end of the world as an expression of a search for one’s past".

[15] An obituary for Robinson in The Daily Telegraph in 1988 reflected on the novel as having "one of the most touching and satisfying denouements in children's literature", and regarded it as a modern classic.

[17] The novel experienced another surge of popularity beginning in 2014, when an animated adaptation by Studio Ghibli brought new attention to the story.

It was dramatised by Beaty Rubens and directed by David Hunter, and included voices of Georgina Hagen (Anna) and Juliet Aaltonen (Marnie).