In 1903, Amundsen led the first expedition to successfully traverse Canada's Northwest Passage between the Atlantic and Pacific Oceans (something explorers had been attempting since the days of Christopher Columbus, John Cabot, Jacques Cartier, and Henry Hudson), with six others in a 47 ton steel seal hunting vessel, Gjøa. Amundsen had the ship outfitted with a small gasoline engine. They travelled via Baffin Bay, Lancaster and Peel Sounds, and James Ross, Simpson and Rae Straits and spent two winters near King William Island in what is today Gjoa Haven, Nunavut, Canada. During this time Amundsen learned from the local Netsilik people about Arctic survival skills that would later prove useful. For example, he learned to use sled dogs and to wear animal skins in lieu of heavy, woolen parkas. After a third winter trapped in the ice, Amundsen was able to navigate a passage into the Beaufort Sea after which he cleared into the Bering Strait, thus having successfully navigated the Northwest Passage. Continuing to the south of Victoria Island, the ship cleared the Canadian Arctic Archipelago on August 17, 1905, but had to stop for the winter before going on to Nome on the Alaska Territory's Pacific coast. Five hundred miles (800 km) away, Eagle City, Alaska, had a telegraph station; Amundsen travelled there (and back) overland to wire a success message (collect) on December 5, 1905. Nome was reached in 1906. Due to water as shallow as 3 ft (0.91 m), a larger ship could never have used the route.

It was at this time that Amundsen received news that Norway had formally become independent of Sweden and had a new king. Amundsen sent the new King Haakon VII news that it "was a great achievement for Norway." He hoped to do more he said and signed it "Your loyal subject, Roald Amundsen."

After crossing the Northwest Passage, Amundsen made plans to go to the North Pole and explore the North Polar Basin. Amundsen had problems and hesitation raising funds for the departure and upon hearing in 1909 that first Frederick Cook and then Robert Peary claimed the Pole, he decided to reroute to Antarctica. However, he did not make these plans known and misled both the Englishman, Robert F. Scott and the Norwegians. Using the ship Fram ("Forward"), earlier used by Fridtjof Nansen, he left Norway for the south, leaving Oslo on June 3, 1910. At Madeira, Amundsen alerted his men that they would be heading to Antarctica in addition to sending a telegram to Scott notifying him simply: "BEG TO INFORM YOU FRAM PROCEEDING ANTARCTIC -- AMUNDSEN." The expedition arrived at the eastern edge of Ross Ice Shelf at a large inlet called the Bay of Whales on January 14, 1911, where Amundsen located his base camp and named it Framheim. Further, Amundsen eschewed the heavy wool clothing worn on earlier Antarctic attempts in favour of Eskimo-style skins. Using skis and dog sleds for transportation Amundsen and his men created supply depots at 80°, 81° and 82° South on the Barrier, along a line directly south to the Pole. Amundsen also planned to kill some of his dogs on the way and use them as a source for fresh meat. A premature attempt, which included Hjalmar Johansen, Kristian Prestrud and Jørgen Stubberud, set out on 8 September, 1911, but had to be abandoned due to extreme temperatures. The painful retreat caused a tempering quarrel within the group, with the result that Johansen and others were sent to explore King Edward VII Land. A second attempt with a team, consisting of Olav Bjaaland, Helmer Hanssen, Sverre Hassel, Oscar Wisting, and Amundsen himself, departed on October 19, 1911. They took four sledges and 52 dogs. Using a route along the previously unknown Axel Heiberg Glacier they arrived at the edge of the Polar Plateau on November 21 after a four-day climb. On December 14, 1911, the team of six, with 16 dogs, arrived at the Pole (90°00'S). They arrived 35 days before Scott’s group. Amundsen named their South Pole camp Polheim, “Home on the Pole.” Amundsen renamed the Antarctic Plateau as King Haakon VII’s Plateau. They left a small tent and letter stating their accomplishment, in case they did not return safely to Framheim. The team returned to Framheim on January 25, 1912, with 11 dogs. Amundsen’s success was publicly announced on March 7, 1912, when he arrived at Hobart, Australia.

Amundsen’s expedition benefited from careful preparation, good equipment, appropriate clothing, a simple primary task (Amundsen did no surveying on his route south and is known to have taken only two photographs), an understanding of dogs and their handling, and the effective use of skis. In contrast to the misfortunes of Scott’s team, the Amundsen’s trek proved rather smooth and uneventful. In Amundsen’s own words:

"I may say that this is the greatest factor -- the way in which the expedition is equipped -- the way in which every difficulty is foreseen, and precautions taken for meeting or avoiding it. Victory awaits him who has everything in order -- luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck."

--from The South Pole, by Roald Amundsen.

Amundsen wrote about the expedition in The South Pole: an account of the Norwegian Antarctic Expedition in the "Fram", 1910-12, published in 1912.

In 1918, Amundsen began an expedition with a new ship Maud, which was to last until 1925. Maud sailed West to East through the Northeast Passage, now called the Northern Route (1918-1920). Amundsen planned to freeze the Maud into the polar ice cap and drift towards the North Pole (as Nansen had done with the Fram), but in this he was not successful. However, the scientific results of the expedition, mainly the work of Harald Sverdrup, were of considerable value. Many of these carefully collected scientific data had been lost during the ill-fated journey of Peter Tessem and Paul Knutsen, two crew members sent on a mission by Amundsen, but they were later retrieved by Russian scientist Nikolay Urvantsev as they lay abandoned on the Kara Sea shores.

In 1923, Amundsen and Oskar Omdal, of the Royal Norwegian Navy, attempted to fly from Wainwright, Alaska, to Spitsbergen across the North Pole. Amundsen and Omdal's aircraft was damaged and they abandoned the journey. In 1925, accompanied by Lincoln Ellsworth, pilot Hjalmar Riiser-Larsen, and three other team members, Amundsen took two Dornier Do J flying boats, the N-24 and N-25 to 87° 44' north. It was the northernmost latitude reached by plane up to that time. The planes landed a few miles apart without radio contact, yet the crews managed to reunite. One of the aircraft, the N-24 was damaged. Amundsen and his crew worked for over three weeks to clean up an airstrip to take off from ice. They shoveled 600 tons of ice while consuming only one pound (400 g) of daily food rations. In the end, six crew members were packed into the N-25. In a remarkable feat, Riiser-Larsen took off, and they barely became airborne over the cracking ice. They returned triumphant when everyone thought they had been lost forever.

In 1926, Amundsen and fifteen other men (including Ellsworth, Riiser-Larsen, Oscar Wisting, and the Italian air crew led by aeronautical engineer Umberto Nobile) made the first crossing of the Arctic in the airship Norge designed by Nobile. They left Spitsbergen on 11 May 1926, and they landed in Alaska two days later. The three previous claims to have arrived at the North Pole – by Frederick Cook in 1908; Robert Peary in 1909; and Richard Evelyn Byrd in 1926 (just a few days before the Norge) – are all disputed, as being either of dubious accuracy or outright fraud. Some of those disputing these earlier claims therefore consider the crew of the Norge to be the first verified explorers to have reached the North Pole. If the Norge expedition was actually the first to the North Pole, Amundsen and Oscar Wisting would therefore be the first persons to attain each geographical pole, by ground or by air, as the case may be.

Amundsen disappeared on June 18, 1928, while flying on a rescue mission with Norwegian pilot Leif Dietrichson, French pilot Rene Guilbaud, and three more Frenchmen, looking for missing members of Nobile's crew, whose new airship Italia had crashed while returning from the North Pole. Afterwards, a wing-float and bottom gasoline tank from the French Latham 47 flying boat he was in, improvised into a replacement wing-float, was found near the Tromsø coast. It is believed that the plane crashed in fog in the Barents Sea, and that Amundsen was killed in the crash, or died shortly afterwards. His body was never found. The search for Amundsen was called off in September by the Norwegian Government. In 2003 it was suggested that the plane went down northwest of Bear Island. Both in 2004 and in late August 2009 an unsuccessful search was made by the Royal Norwegian Navy for the wreckage of Amundsen's plane, using the unmanned submarine Hugin 1000. The search focused on a 40 square mile area of the sea floor, and was documented by the German production company ContextTV.