Among Johnson's works are collaborative efforts such as Let It Snow, a holiday romance novel of interwoven stories co-written with John Green and Lauren Myracle, and a series of novellas found in New York Times bestselling anthologies The Bane Chronicles, Tales from the Shadowhunter Academy, and Ghosts of the Shadow Market.

[1] Johnson later worked variously as literary manager of a Philadelphia theater company, a waitress in a theme restaurant, a secretary, a bartender in Piccadilly, and an occasional performer in New York City.

Johnson herself has said she considers Ginny's adventure to be "a little bit of a fairy tale," and has cited the song "Charmed Life" by Irish artist Neil Hannon of The Divine Comedy for its likeness to the spirit with which the novel was written.

Suite Scarlett was selected by the American Library Association for their list Best Books for Young Adults 2009, and received a starred review by Booklist.

[citation needed] On November 27, 2009, Johnson first became a New York Times bestselling author when Let It Snow: Three Holiday Romances (October 2008, Speak) reached number ten on the Children's Paperback list.

[8] A holiday romance novel of interwoven stories co-written with John Green and Lauren Myracle, Let It Snow begins with Johnson's "The Jubilee Express".

Meanwhile in 2011, Johnson became the ongoing lead coordinator of LeakyCon's Lit Track programming, the literary focused experience for fans of young adult fiction more broadly, a production of Mischief Management.

[10] In March 2014, Johnson was selected to represent the YA category for World Book Day in the United Kingdom[11] with the first publication of her novella The Boy in the Smoke, a prequel story to the Shades of London series.

[12] That August, The Boy in the Smoke was additionally released to an international audience for free through the online reading and story writing platform Wattpad.

Johnson has maintained a personal website about her work and experiences as an author since 2005, with blog entries dating as far back as August of that year.

In June 2008, she joined then new micro-blogging platform Twitter as one of its early users, where she has likened her many posts about everything from the news of the day to searching the town of Guildford for a baking pan[16] to the habit she had developed of leaving post-it notes around for others to read while working in theater productions.

Johnson provided the foreword for The Great Glowing Coils of the Universe (Harper Perennial, 2016), a collected volume of early podcast scripts.

"[27] According to a May 2023 Town Watch episode of Says Who?, shortly after moving to New York City, Johnson got a job as a server in the Jekyll & Hyde Club family of restaurants and pubs.