Ms Marin, who was just 37 years old when she took office in 2019, led a five-party centre-left governing coalition until April of this year.
She received praise for her Cabinet’s handling of the Covid-19 pandemic, and her vocal support of Ukraine in the last year had increased her international visibility.
She also saw her country become the 31st member of Nato in April.
But on Thursday, as an MP, she announced it was “time to move on”.
“I am eager to step into a new role. I also believe that it can benefit the whole of Finland,” Finnish public broadcaster YLE quoted her as saying.
Ms Marin will join the Tony Blair Institute for Global Change as a strategic counsellor. The non-profit helps governments and leaders internationally by advising on strategy, policy and delivery.
“The task will be to be an adviser to different countries, governments and leaders on policy issues that are familiar to me, such as good governance, technology, climate, gender equality and other issues that I have had to work with,” she said according to the Huvudstadsbladet newspaper, one of Finland’s largest.
She told a press conference as she announced her departure from politics: “I believe that I can serve those voters (in Finland) well and maybe even better in the new assignment.”
Tony Blair said of her appointment: “Ms Marin is bold and practical, understands the role that technology can play as the enabler of a more efficient and citizen-centred state, and will lend her experience to our mission in more countries throughout the world.”
In the April parliamentary elections, she lost to Finland’s main conservative party in a tight three-way race that saw right-wing populists take second place.
Ms Marin’s Social Democrats ended in third, dashing her hopes for re-election.
As prime minister she headed a coalition government, including her own Social Democrats, the centrist Centre Party, the Green League, the Left Alliance and the Swedish People’s Party in Finland.
Her private life and active social media use received a lot of international coverage during her premiership.
In August 2022, she apologised after the publication of a photo that showed two women kissing and posing topless at the official summer residence of the country’s leader.
The photo came out after a video that showed Ms Marin dancing and singing with friends prompted a debate about whether a prime minister in office is entitled to party. She took a drugs test to prove she was not under the influence.
In May Ms Sanna announced on Instragram that she and her husband Markus Raikkonen were divorcing after 19 years together.