Mariko Iwadate was born on 8 February 1957 in Sapporo, Hokkaido, Japan. She is a Japanese manga artist whose career in the industry began at a young age. Iwadate made her professional debut as a manga creator in 1973 with the short story Rakudai Shimasu, published in the shōjo manga magazine Margaret. Following her debut, she became a regular contributor to Margaret and later to the magazine Young You, establishing herself as a consistent voice in the shōjo genre throughout the 1970s and 1980s. Her early works from the 1970s are considered part of a movement within shōjo manga known as otomechikku. This style focused on the everyday lives, romantic experiences, and psychological development of Japanese high school girls, featuring a dreamy and distinctly feminine aesthetic.

During the 1980s, the themes in Iwadate's work began to evolve, exploring more complex and deeper subject matter. The short story Angel, published in 1982, is noted as a significant stylistic turning point in her career, with narratives increasingly focusing on family relationships and the nuances of adult life. One of her most notable works is the manga Ichigatsu ni wa Christmas, also known as Christmas in January. Originally serialized in Margaret in 1983, this story was later adapted into a single-episode anime OVA in 1991. The OVA, directed by Tetsu Dezaki and produced by the studio Magic Bus, is a character study centered on a young woman with a traumatic past who develops feelings for her neighbor. The adaptation lists Iwadate as the original creator, bringing her manga to an audience beyond print. In addition to this adaptation, she received significant recognition for her series Uchi no Mama ga iu Koto ni wa, which won the prestigious Kodansha Manga Award in the shōjo category in 1992.

Iwadate is recognized for a distinctive artistic identity that blends a romantic visual style with literary substance. Her work is described as a combination of literature and shōjo manga, and she has been cited as an influence on acclaimed writer Banana Yoshimoto. Critically, her panel layouts and use of white space are noted for their boldness in conveying emotion and creating a sense of uncertainty within the frame. Throughout her career, she has produced a substantial body of work, including series such as Futari no Dōwa, Garasu no hanataba ni shite, Reizōko ni Pineapple Pie, Kodomo ha nan demo shitte iru, Kirara no Ki, and Amaririsu, many of which were published by Shueisha. Her long-running series often continued for several years, demonstrating her sustained presence in the shōjo and josei manga markets.