Spring was born on 21 February 1996. She has a sister. Spring reports an interest in journalism from age eight, which included watching BBC World News while on holiday. She attended Sutton High School, London, and while there she was involved in a programme run by Newsquest for young journalists, winning an award for best news article of 2011 by a Year Eleven student. She studied French and Russian at Pembroke College, Oxford (matriculating in 2014) and wrote for and edited the student newspaper Cherwell. Spring won the Ronnie Payne Prize for Outstanding Foreign Reporting in 2017. She spent her year abroad in Yaroslavl (Russia), and Paris, contributing news articles to The Moscow Times, The Local, and Le Tarn Libre. Spring undertook work experience at The Guardian and Private Eye. After graduation, she applied for various journalism programmes including at the BBC but was not successful. Senior news reporter for The Guardian Alexandra Topping suggested that Spring contact various BBC journalists that she admired. Emily Maitlis replied to Spring and gave her an opportunity to work on Newsnight. By the end of 2018, Marianna had co-produced a video for Newsnight, about protesters from across the French political spectrum joining the gilets jaunes.
In March 2020, she was appointed the BBC's first specialist disinformation and social media reporter which followed establishment of similar roles at American news organisations such as CNN and NBC. In 2021 Spring started to work as a reporter for the current affairs documentary programme Panorama and was selected by Forbes as one of their Media and Marketing "30 Under 30" in 2021.
She was promoted to correspondent in August 2022. Spring was nominated as Young Talent of the Year at the Royal Television Society's Journalism Awards in 2023. Her debut book Among the Trolls: Notes From the Disinformation Wars is due for publication by Atlantic Books in autumn 2023.
"The focus of my job is to humanise disinformation and explain its impact to viewers, listeners and readers." --Spring describing her role in March 2021.