Einar Gerhardsen was born in the municipality of Asker, in the county of Akershus. His parents were Gerhard Olsen (1867 – 1949) and Emma Hansen (1872 – 1949). He was married to Werna and they had two sons Truls and Rune Gerhardsen and a daughter Torgunn. His brother was Rolf Gerhardsen and with him Einar Gerhardsen also had a lifelong working relationship. From the age of 17, Gerhardsen went to meetings in the Labour party's youth movement. Originally a road worker, Gerhardsen became politically active in the socialist labour movement during the 1920s. He was convicted several times of taking part in subversive activities until he, along with the rest of the Labour party, gradually moved from communism to democratic socialism. He participated in the Left Communist Youth League's military strike action of 1924. He was convicted for assisting in this crime and sentenced to 75 days of prison.

By the middle of the 1930s Labour was a major force on the national political scene, with Gerhardsen as the Mayor of Oslo and Johan Nygaardsvold as Prime Minister of a minority cabinet. During World War II, Gerhardsen took part in the organised resistance against Nazi occupation and was arrested on 11 September 1941 and interned in concentration camps at Grini in Norway and at Sachsenhausen in Germany in August, 1944. After the war, Gerhardsen formed the interim government which sat from the end of the occupation in May 1945 until the elections held in October the same year. The elections gave Labour an absolute majority in Parliament, the Storting, which it retained until 1961. Gerhardsen served as President of the Storting from January 10, 1954 to January 22, 1955.

During and after his periods in office he was greatly respected by the people, even those not sharing his social democratic views. The administrations he led forged an eclectic economic policy in which government regulation of commerce, industry and banking was combined with market economics. Abject poverty and unemployment were sharply reduced by his government's policies of industrialisation and redistribution of wealth through progressive taxation, together with the creation of a comprehensive social security system. In foreign policy, he aligned Norway with the Western powers at the end of the 1940s after some initial hesitation within the governing party, and Norway became a founding member of NATO.

Documents from 1958 reveal that the Gerhardsen's government knew that Israel was going to use heavy water supplied by Noratom AS for plutonium production, making it possible for Israel to produce nuclear weapons.

In November 1962 an accident in which several miners died occurred in the Kings Bay coal mine on Spitsbergen in the Svalbard archipelago. In the aftermath, the Gerhardsen government was accused of not complying with laws enacted by parliament. In the summer of 1963 a vote of no confidence passed with the support of the Socialist People's Party and a centre right minority coalition government was formed, under John Lyng. Although this new government lasted only three weeks, until the Socialist People's Party realigned itself with Labour, it formed the basis for an opposition victory under the leadership of Per Borten at the 1965 elections. Gerhardsen retired from national politics in 1969 but continued to influence public opinion through writing and speeches.

Gerhardsen's political legacy is still an important force in Norwegian politics, especially within his own party, although some of the social policies of his government have been revised.