Kathleen Peyton, who has died aged 94, was a prolific writer for children under the pseudonym KM Peyton. She was best known for the highly regarded and popular Flambards series, which was adapted for television in 1978.

In writing Flambards (1967), The Edge of the Cloud, Flambards in Summer (both 1969) and Flambards Divided (1981), Kathy drew on the farming countryside around her home in Essex and her love of horses and particularly of hunting. Set at the beginning of the first world war, in the farmhouse where a once-wealthy family live a fractured and angry life, the Flambards series captured the profound social changes brought by the war.

The story begins when Christina, a 13-year-old orphan, arrives to stay with her bad-tempered Uncle Russell and his sons, Mark and William, knowing little about her relatives and nothing about their passions – horses, hunting and flying. Over the years and the series, her relationships with her Russell cousins, rough-cut Mark and sensitive William, change from a teenager’s infatuation to something more serious. Kathy infused Christina’s complicated and developing feelings with intensity and a naive romanticism that was suitable for her young readers.

She was a fluent storyteller with a great gift for character and emotions, drama and place. Many of her adventures followed well-intentioned children taking on social injustice and the adults who perpetrate it. With a lot of action, the books were tightly plotted, and her teenage characters had enough emotional complexity to be captivating to readers.

The Flambards series made Kathy’s reputation as a significant writer for teenagers – although she had originally intended them to be for adults. Despite the vitriolic letters she received from mothers of teenage girls who were outraged by their sexual undertones, the books were an international success and she won the 1969 Carnegie medal for Flambards and the 1970 Guardian Children’s Book award for The Edge of the Cloud, a terrifying but heady story about the allure of aviation.

Kathy showed the same confidence in writing about teenagers in a contemporary, school-set series about the complicated life of Pennington, a gifted but troubled young pianist. Inspired by a handsome boy Kathy had seen on a train, who later saved a man from drowning, Pennington’s Seventeenth Summer (1970), The Beethoven Medal (1970) and Pennington’s Heir (1973) were fuelled by the same delightful naive romanticism as Flambards.

Although many of her works were about horses and riding, including Who, Sir? Me, Sir? (1983), another success which was turned into a TV series, she could never be categorised as just a writer of pony stories. Among her 70 books were thrillers, including Prove Yourself a Hero (1977) and Midsummer Night’s Death (1978), and stories about conservation and ghosts. She spent 20 years researching Dear Fred (1981), based on the life of the jockey Fred Archer. She was appointed MBE in 2014.

The daughter of Ivy (nee Weston) and Joe Herald, Kathy was born in Birmingham, where her father worked as a water engineer. Travelling with his job, the family moved to London, where Kathy went to Wimbledon high school. She wrote her first book, Sabre, the Horse from the Sea (1948), when she was 15, and it was published while she was still a teenager.

She gained a place at Oxford University although, much to her school’s displeasure but with her parents’ approval, she turned it down to go to art school. She started at Kingston in west London, but, when her father’s job moved to Manchester, she transferred to Manchester School of Art and gained an art teacher’s diploma in 1951.

In 1950, Kathy eloped with Michael Peyton, an older student who had returned to his studies after second world war service, including time as a prisoner of war. After graduating, Kathy taught at Northampton high school for three years, but she had already begun a writing career with Michael. A keen sailor and adventurer, he contributed to the plots of the simple adventure stories they wrote for Boy Scout magazine under the authorship of K and M Peyton. Three books developed out of the magazine stories. On the publication of the first, North to Adventure (1958), the publisher opted for a single author: KM Peyton.

Adopting the authorial name, Kathy set about what she called “a proper book”, as Michael moved on to other things. Windfall (1962), The Maplin Bird (1964) and Thunder in the Sky (1968) were all historical novels drawing on the Essex countryside, but it was only with Flambards that she added horses.

Kathy wrote confidently and fast, revising little and she did not take kindly to editorial suggestions. She never ran out of ideas and was still writing until a year ago. She continued to ride into her 80s, was a keen and successful gardener, and had a great gift for friendship across all ages.

Michael died in 2017. Kathy is survived by their daughters, Hilary and Veronica.

• KM Peyton (Kathleen Wendy Peyton), children’s writer, born 2 August 1929; died 19 December 2023