Along with Helmer Hanssen, Hassel was picked as an expert dog driver to take part in Roald Amundsen's South Pole expedition 1910 – 1912. On 14 December 1911, Hassel together with Amundsen, Helmer Hanssen, Olav Bjaaland and Oscar Wisting were the first to reach the South Pole. For his participation in the expedition, he was awarded the South Pole Medal ("Sydpolsmedaljen"), the Royal Norwegian award instituted by King Haakon VII in 1912 to reward participants in Roald Amundsen's South Pole expedition.