Becoming a member of the World War II coalition government in 1944, Erlander rose unexpectedly to the leadership upon the death of Prime Minister Per Albin Hansson in October 1946, maintaining the position of the Social Democrats as the dominant party in the country.

[14] In the 1968 general election, he won his seventh and most successful victory, with the Social Democrats winning an absolute majority of the popular vote and seats in the lower chamber.

[30] He also met and studied natural sciences with fellow student and future physicist Torsten Gustafson, who would later serve as an advisor on nuclear affairs to Erlander during his premiership.

[31][32] In addition to his scientific studies, Erlander also read some economics,[33] and was an active member of Wermlands Nation, where he was elected Kurator (head executive) in 1922.

[51][2] Erlander and his wife were in Lund when Hansson died, and when they returned to the Grand Hôtel, they were informed of his death by Minister of Defense Allan Vougt.

[57] In the two years leading up to the 1948 election, Erlander visited numerous Social Democratic organizations across the country to solidify his support and explain party policies.

[62] Minister of Commerce Gunnar Myrdal implemented policies such as selling the Soviet Union machinery on a fifteen-year credit and a 17% appreciation of the Swedish krona.

[5] Following this election, the Social Democrats remained in power, but desired to maintain a long-term majority, so they offered to form a coalition government with the Centre Party.

[128] Undén's successor, Foreign Minister Torsten Nilsson informed him via telephone the day of the arrest while Erlander was in a restaurant in Italy on vacation with his wife, and asked him to return to Sweden immediately.

Erlander stated that he would regard votes of censure as a question of confidence in his entire cabinet, and that it was "a tragedy" that Wennerström's arrest and trial became a political issue.

Olof Palme, now Minister for Communications (Transport), oversaw the project,[141] which was often seen as a way to bring Sweden in line with the driving standards of most of Europe.

His popularity increased as television began to play an important role in Swedish politics, as Erlander's amiable and humorous personality was more apparent.

He became known as "Sweden's longest Prime Minister" referring to both his physical stature – 192 cm (6 ft 4 in)[b][k] – and his record tenure of 23 years (the Swedish word lång meaning both 'long' and 'tall').

The expected deficit was $180 million, and to prevent it from increasing, Erlander's government proposed ending deductions of old-age pension fees from taxable income.

In order to maintain employment for his vast electorate and Swedish sovereignty as a non-NATO member, the armed forces was greatly expanded, reaching an impressive level by the 1960s, while nuclear capability was ultimately dropped after outcries, not least from the Social Democratic Women's League.

The question of nuclear weapons as a means to deter a possible attack remained a divisive factor in Swedish society and among Social Democrats and prompted diplomatic agreements with the United States, guaranteeing intervention in the case of an invasion.

Some international observers speculated that Erlander and future Swedish leaders maintained interest in a hypothetical nuclear system for defense, but did not take action to develop one.

The proposal fell apart and was shelved in January 1949 due to Norwegian resistance and the country's acceptance of membership in NATO, with Denmark and Iceland following suit.

[196] Erlander was generally considered a pro-Western leader despite this, and wrote that America was doing Europe a great service by allowing itself to increase their arms for defense against the Soviet Union.

[197] In 1961, Erlander and President John F. Kennedy advocated for the West to strengthen the United Nations and its Secretary General, Swedish politician Dag Hammarskjöld.

[198] Erlander was a strong supporter of the proposed Nordic economic community Nordek, and held meetings on the subject with Finnish President Urho Kekkonen and Prime Minister Mauno Koivisto in 1969.

[208] The Soviet Union's documents stated that Wallenberg died in a cell in 1947 of a heart attack, but Erlander, the Swedish government, and international observers were skeptical.

[213] Khrushchev continued denying that Wallenberg was in the Soviet Union, and Erlander and the government expressed "deep disappointment" over the lack of development in the case.

[214] There were anti-Khrushchev protests in Sweden from Soviet exiles upon his visit, and the Swedish press criticized him as a liar relating to his discussions over Wallenberg and the stringent security (3000 police officers upon his arrival[208]) around him.

After the Stockholm ceremony, his funeral crossed the country and returned to his home town of Ransäter, Värmland, in a triumphant procession for the final rest.

[30] Despite Erlander being familiar with the writings of Karl Marx and identifying as a socialist, he did not subscribe to full Marxism and did not support nationalization, instead believing in a strong public sector under well-regulated capitalism with social welfare programs.

[245] Through one Erlander's Finnish ancestors, Simon Larsson (née Kauttoinen) (c.1605-1696), he is a distant relative of Stefan Löfven, the Social Democratic Prime Minister of Sweden from 2014 to 2021.

[20][27] Ruin notes that as Sweden encountered difficulties in the 1970s, nostalgia sometimes influenced positive views of Erlander, and that his time as leader was looked upon by some as a "golden age" of Swedish history.

[265] Left Party leader Nooshi Dadgostar praised Erlander in 2022, citing him as an inspiration who passed reforms laying the foundation of Sweden's welfare state.

[276] In the 2021 series En Kunglig Affär [sv], which depicted the Haijby scandal, Erlander was portrayed by Swedish actor Emil Almén.