But he described his time in prison as a happy and productive period that allowed him to formulate his life goals.
After completing his studies, Park went into law and defended many political activists in the 1980s and 1990s.
He helped launch the People's Solidarity for Participatory Democracy, an influential NGO pushing to reform the conglomerates that dominate South Korean business.
Park also founded the Beautiful Foundation - a philanthropic group that promotes volunteerism and community service - in the hopes of giving people the ability "to feel wonderful and beautiful" by donating to its cause.
It grew into one of the largest non-profit organisations in South Korea and launched the Beautiful Stores, a chain of charity shops modelled after Britain's Oxfam shops.
Bespectacled and soft-featured, Park was considered a political novice when he first entered the Seoul mayoral race as an independent candidate in 2011 but he defeated Na Kyung-won, the most prominent female lawmaker for the then ruling conservative party.
He subsequently joined the Democratic party and made a name for himself as a three-term mayor of the South Korean capital - his final victory coming in a 2018 landslide when he took more than 50% of the vote.
During his campaign, Park - known for his hardworking and down-to-earth character - pledged to create a cleaner city that ensured gender equality, and to bring about peace between North and South Korea.
On his official Twitter account, Park introduced himself as a "Social Designer", saying: "These days, I... dream of creating Seoul that puts the people first. Please join me in this dream."
But he also earned a reputation as a workaholic boss, with reports saying a handful of civil servants at the Seoul city government had committed suicide due to work-related stress under him.
Earlier this week he courted controversy by being one of several senior Democratic party politicians to attend the funeral of the mother of Ahn Hee-jung, a former provincial governor convicted of sexual offences last year.