After attending primary school, Koivisto worked a number of jobs, and at the beginning of the Winter War in 1939 joined a field firefighting unit at the age of 16.

In 1949, communist-controlled trade unions attempted to topple Karl-August Fagerholm's social democratic minority government, and the Social Democratic leadership of the Finnish Confederation of Trade Unions (SAK) declared the port of Hanko an "open site", urging port workers who supported legality to go there.

The Communist newspapers branded Koivisto as their number one enemy due to his status as a major figure in the struggle for control of the trade unions.

He served as prime minister for two years until the 1970 parliamentary election, which saw the other parties in the coalition government—Centre, SKDL, SPP, and TPSL—suffer heavy losses, bringing about Koivisto's resignation.

By this point there was increasing dissatisfaction with the aging President Kekkonen, whose failing health was becoming difficult to conceal, and also a perceived lack of change.

As prime minister and chairman of the Bank of Finland who enjoyed high ratings in opinion polls, Koivisto began to be seen as a likely future candidate for the presidency.

In early 1981, President Kekkonen began to regret Koivisto's appointment as prime minister and started to offer support to those who wanted to get rid of him.

Some question whether this government crisis was just a part of the ruthless "presidential game" that top politicians such as Koivisto and Social Democratic chairman Sorsa were playing with one another.

To a journalist's question, intended to be a difficult one, on the issue of relations with the Soviet Union, Koivisto replied that they were nothing to boast about; this answer increased his popularity.

Koivisto won 167 of the 301 votes of the electoral college in the first round; his closest competitor, NCP candidate Harri Holkeri, received 58.

His victory completed a long process of integrating the Social Democrats into Finland's political life, dating back to the Civil War.

As president, Koivisto kept a low profile and used less authoritarian leadership tactics than Kekkonen had employed, refraining from using some of his presidential powers and initiating a new era of parliamentarianism in Finland.

[citation needed] Mauno Koivisto's presidency (1982–1994) marked a deliberate shift from the strong presidential dominance of his predecessor, Urho Kekkonen, towards a more parliament-centered governance.

He consciously limited the exercise of presidential powers, fostering an environment where the Prime Minister and Parliament assumed greater responsibility in decision-making processes.

[10] Koivisto also supported constitutional reforms aimed at reducing presidential authority, thereby strengthening Finland's parliamentary system.

[12][13][14] As the leader of Finland's foreign policies he initially continued Kekkonen's line until the collapse of the Soviet Union.

[19][20] Koivisto made two bold unilateral diplomatic moves that significantly changed the Finnish political position.

In 1990, after the reunification of Germany, Koivisto unilaterally renounced the terms of the Paris Peace Treaties which limited the strength and armament of the Finnish Defence Forces.

The other major move was the renunciation of the Finno-Soviet Treaty (Finnish: YYA-sopimus) in 1991, concurrently with the fall of Soviet Union.

Koivisto's health deteriorated in December 2016 due to Alzheimer's disease and his wife Tellervo started as his caregiver.