Karlsson won gold in the 50 km event at the 1948 Winter Olympics and nine Vasaloppet victories.

For his success in cross-country skiing, Karlsson was awarded the Holmenkollen medal in 1952 (shared with Stein Eriksen, Torbjørn Falkanger and Heikki Hasu).

With the aid of Sven Jerring's radio sports commentary, he made Vasaloppet known throughout the country.

[1] Between 1954 and 1966 Karlsson trained the national cross-country skiing team, yet he lived all his life in his native village Östnor near Mora, where a road was named after him during his lifetime, and where he ran the sport equipment store "Mora-Nisse Sports".

The Mora-Nisse's Jubilee Fund was established on his 65th birthday, which supports young Swedish skiers.