Amy Roberta "Berta" Ruck was born in the Punjab in India, where her father, Colonel Arthur Ruck, was serving in the British army. Not long afterwards, the family moved to North Wales, where she went to boarding school in Bangor.
After graduation, she attended art schools in London and Paris and worked as a magazine illustrator. She began to write and publish short stories for adults and children. In 1909, she married Oliver Onions, a fellow writer, with whom she had two sons. With his help, she expanded her 1912 short story "His Official Fiancée," which had appeared in the periodical Home Chat, into book form. It was published two years later in both the UK and the USA, and its success launched her career as a prolific and popular writer. She produced numerous short stories and more than 80 romance novels over the next 60 years. Some of them, including His Official Fiancée and Sir or Madame? (1923) were also adapted into films. She also wrote five autobiographical works, including Ancestral Voices (1972), her last book.
In 1922, her name was inadvertently included in the novel Jacob's Room by Virginia Woolf, apparently chosen at random by the author.