But that wasn't all that drew global attention: she was in a coalition government made up of five parties all led by women.
Stepping into the new government with Marin was Li Andersson (Left Alliance), 32; Maria Ohisalo (Green League), 34; Anna-Maja Henriksson (Swedish People's Party of Finland), 55; and Katri Kulmuni (Centre Party), 32.
So, a year on, how has the female-led government done?
Finland 'interested in what other people think about us'
Although women remain highly underrepresented in governments worldwide, in Finland, Marin is the fourth female prime minister.
“We have a tradition of female leaders in politics, so it wasn’t very exceptional,” said Johanna Kantola, a professor of gender studies at Tampere University, who said Finns started to realise it was special following the “international hype”.
“We are a small country so we are very interested in what other people think about us. Equality is part of the country’s identity so people followed very much what this government was doing.”
Marin comes from a working class background and was elected to the Tampere City Council at the age of 27. She was first elected to Finland's parliament in 2015.
Leader of the Social Democratic Party, she came into office following a postal strike that forced the previous prime minister and party leader Antti Rinne to resign.
Like the government that was elected in April 2019 under Rinne, Marin has an agenda that includes progressive policies on gender equality and climate change.
But COVID-19 has been an early challenge for the five party leaders, each of whom are government ministers.
“This first year is the year of the pandemic. So the most policies they have been forced to do are related to the fight of this COVID pandemic,” said Kimmo Elo, a senior researcher in European Studies at the University of Turku, which means that many of the government's policy plans haven't yet been carried out.
An efficient and effective early response
Finland's government had been praised for its early response to the COVID-19 crisis. Unlike its neighbour Sweden which kept most businesses open, Finland closed schools and locked down the country on March 16 with just under 300 recorded cases of the virus.
For many experts, it's on a list of countries that seemed to react well early on, in part due to clear and early actions to tackle the pandemic.
“What made the response good in the beginning and what was kind of attributed to the prime minister... was this kind of very clear communication skill,” said Kantola, who said Marin also relied on “outside expertise” to help her manage the crisis.
Finland initially brought down COVID-19 infections to under 20 new cases a day over July and August but is now in the midst of a second wave.
Yet Marin’s leadership has been linked to other female leaders in anecdotes and studies suggesting that countries run by women prevented more infections and deaths.
A study by researchers at the University in Reading and Liverpool found that there was a “significant and systemic” difference in the number of COVID-related deaths and cases in countries with female leaders.
This was in part because “women leaders reacted more quickly and decisively in the face of potential fatalities”.
The study goes on to analyse women leaders’ communication style and weighing of risk to human life versus risk to the economy.
Second wave tests the popularity of Marin's government
Despite those early successes, the Marin government now faces stronger criticism over its COVID-19 response.
Finland is in the midst of a second wave of COVID-19 with hundreds of infections a day. So far, there have been fewer deaths than during the first wave.
The country’s Chancellor of Justice also said last week Finland’s ministries’ response, organisation and cooperation were insufficient to fight the pandemic.
“Based on the explanations received by the Chancellor of Justice, inter-ministerial co-operation did not initially work effectively and the division of responsibilities within the Ministry of Social Affairs and Health was not clear enough,” Chancellor of Justice Tuomas Poysti said in a statement.
This comes as the government’s support has fallen in polls, with national broadcaster YLE showing the populist Finns Party rising to surpass the Social Democrats in popularity.
Some of this is a return to "normal", say experts, explaining the popularity the government enjoyed in March was more of an outlier from normal politics.
“Since the mid-summer, satisfaction with government parties has fallen and once again the drop [in popularity] that we had in the spring with regards to populist parties has changed, and they are recovering in opinion polls across Europe,” said Elo at the University of Turku.
"The right-wing opposition fell very much into line in Spring to support the Government’s tackling of the crisis," said Anne Holli, a professor of politics at the University of Helsinki.
But disagreements "between balancing of economic concerns and social policy/public health concerns that have become more exacerbated since summer, as the opposition has also become sharper in its criticism towards the government," she added.
A progressive programme, with some key moves towards gender equality
The Marin government has still made some steps towards more progressive gender policies, experts say.
For one, they brought back Finland’s right to childcare for those under seven, which the previous "right-wing government discontinued in 2016 after 20 years’ of its existence and which has been hailed as the hallmark of the Finnish 'women-friendly welfare state'," said Holli at the University of Helsinki.
In addition, the government has put forward proposals for a new parental leave policy that would give fathers the same amount of parental leave as mothers and gives non-biological parents the same leave as well.
It's a plan that has support as the current system "places too much burden on mothers at home and there needs to be a bigger quota for fathers to stay at home with younger children," said Kantola.
Compared to many other countries, however, it's a generous programme. Many countries give little or no leave for fathers.
Four of Finland's five female party leaders are under the age of 36 -- and are also using social media platforms to better communicate with the people. Marin has posted photos of her family life, including breastfeeding and photos of her home life.
"A lot of women politicians in Finland are using Instagram in very specific ways to share about their pregnancy or having small children and leading a political career, so it’s also a question of controlling this story yourself," said Kantola.
But it doesn't always work out the way people want it to.
In one early misstep last year, Finland's finance minister Katri Kulmuni asked in an Instagram poll whether women and children linked to the so-called Islamic State should be allowed back into the country.
She later apologised for the action that human rights organisations said was "poorly judged".
Progress lacking in some areas
Many experts say it’s tough to judge the current government because they haven’t moved forward on many of their priorities.
"The final outcome where we can assess what is going to happen, we are not there yet, where the real political struggles kick in and where we can see what was achieved," said Kantola.
For instance, the country's transgender policies are not very progressive, experts say, although the government has said they are committed to changing that.
"There’s this talk that it’s important but there’s no progress," Kantola says. At the moment, Finland still requires transgender people to become sterilised after they change genders, a practice that a UN report denounced as "torture".
Meanwhile back in September, the Council of Europe's Commission against Racism and Intolerance also called on Finland to "tackle growing racist and intolerant hate speech" and "better coordinate integration activities for immigrants".
The country's next elections are not planned until April 2023, so if parliament is not dissolved, the government will continue to have the time to pursue its agenda, but will likely do so amid a complicated economic situation due to the COVID-19 pandemic.
For many, however, it remains to be seen how this government will cooperate to do so.
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