Gustaf V was born in Drottningholm Palace in Ekerö, Stockholm County, and at birth was created Duke of Värmland. On 8 December 1907, he succeeded his father to the Swedish throne, which had been separated from the Norwegian throne two years earlier.

On 20 September 1881 he married Princess Victoria of Baden in Karlsruhe, Germany. She was the granddaughter of Princess Sophie of Sweden, and her marriage to Gustaf V united, by a real blood link (and not only so-called adoption), the reigning Bernadotte dynasty with the former royal house of Holstein - Gottorp.

Gustaf V was the last Swedish king to intervene directly in the politics of his country (in 1914 on the disputes over defence budgets). He was a conservative man, who did not approve of the democratic movement and the demands for workers' rights. Gustaf V was also the last Swedish King to be Supreme Commander of the Swedish Armed Forces (between 1907 and 1939).

Gustaf V was considered to have German sympathies during World War I. His political stance during the war was highly influenced by his wife, who felt a strong connection to her German homeland. On 18 December 1914, he sponsored a meeting with the other two Kings of Scandinavia to demonstrate unity within and between them. Another of Gustaf V's objectives with this three King conference was to dispel suspicions that he wanted to bring Sweden into the war on Germany's side.

Both the King and his grandson Prince Gustav Adolf, socialized with certain Nazi leaders before World War II, though arguably for diplomatic purposes. Gustaf V attempted to convince Hitler during a visit to Berlin to soften his persecution of the Jews, according to historian Jörgen Weibull. He was also noted for appealing to the leader of Hungary to save its Jews "in the name of humanity." At the behest of American President Franklin D. Roosevelt, Gustaf V appealed to Hitler for peace negotiations in 1938, "in the interest of peace".

When Nazi Germany invaded the Soviet Union in June 1941, Gustaf V tried to write a private letter to Hitler thanking him for taking care of the "Bolshevik pest" and congratulating him on his "already achieved victories". He was stopped from doing so by the Prime Minister Hansson. Nevertheless, the King sent the message to Hitler (through a telegram by the German embassy in Stockholm) behind the back of the Government.

According to Prime Minister Per Hansson the King had, during a private conversation, threatened to abdicate if the Government did not approve of the German request for permission to transfer one armed division – the Engelbrecht Division – through Swedish territory from northern Norway to northern Finland in June 1941. The accuracy of this claim is debated, and the King's stated intention (if he did in fact make this threat) was to avoid conflict with Germany.

However, confirmation of the King's action is contained in German Foreign Policy documents captured at the end of the war. On 25 June 1941, the German Minister in Stockholm sent a "Most Urgent - Top Secret" message to Berlin in which he stated that the King had just informed him that the transit of German troops would be allowed. He added:

The King's words conveyed the joyful emotion he felt. He had lived through anxious days and had gone far in giving his personal support to the matter. He added confidentially that he had found it necessary to go so far as to mention his abdication.

According to Ernst Wigforss, both Gustaf V and Prince Gustav Adolf attempted to persuade the Swedish Government to allow the Allies to transport troops through Sweden, though this was rejected by the Government because it was felt it would cause retributions from Germany.

Gustaf V was tall and thin. He wore pince-nez eyeglasses and sported a pointed mustache for most of his teen years.

Gustaf V was a devoted tennis player, appearing under the pseudonym Mr G. As a player and promoter of the sport, he was elected into the International Tennis Hall of Fame in 1980. The King learned the sport during a visit in Britain in 1876 and founded Sweden's first tennis club on his return home. In 1936 he founded the King's Club. During his reign, Gustaf was often seen playing on the Riviera. On a visit to Berlin, Gustaf went straight from a meeting with Hitler to a tennis match with the Jewish player Daniel Prenn. During World War II, he interceded to obtain better treatment for Davis Cup stars Jean Borotra of France and Gottfried von Cramm of Germany, who had been imprisoned by the German Government.

Allegations of a homosexual affair made the Court pay Kurt Haijby at least 170,000 crowns. This later led to the Haijby affair. No other evidence has ever been presented to support the allegations.

Gustaf V was the 1,062nd Knight of the Order of the Golden Fleece in Spain, the 828th Knight of the Order of the Garter in 1905 and the 216th Grand Cross of the Order of the Tower and Sword.

King Gustaf V died in Stockholm in 1950.