Lasse Wellander started playing guitar as a youngster in the early sixties and was a member of local bands in his hometown of Nora. Later, he started playing with a group called Blues Quality. When that band split up, the remains formed the nucleus of Nature, which was the backing band on Polar recording artist Ted Gärdestad's summer tour of 1973 and again in 1974. That's how Lasse came to know Björn and Benny.

His first ABBA session was in October 1974 when Intermezzo No.1 and Crazy World were recorded.
Lasse soon took over as main guitarist on ABBA's albums, and toured with them in 1975, 1977, 1979 and 1980. He has continued working with Björn and Benny after the ABBA period, and appeared on the Chess concept album, the two Gemini albums, and also on the 1993 Josefin Nilsson album Shapes.

One of the most frequently used Swedish session musicians, Lasse has also had some solo success with two Top 40 albums in the mid‑1980s. In 1992 he had a big radio hit with his instrumental version of the Chess song Anthem.