Lewis is also the former singer for Aesma Daeva, a band that toured with Therion in Canada and the USA in 2005. In October, 2007, Lewis performed with both bands on the US/Canada portion of Therion's world tour (Aesma Daeva supported Therion).
On September 11, 2011, Christofer Johnsson, Therion's head, announced that Lewis was named permanent member of the band, becoming then the first female member to have permanent status in the history of the band.
As a permanent member of Therion, Lori participated in the recording of Les Fleurs du Mal, dedicated to the 25th anniversary of the band.
On May 10, 2014, the band announced that Lewis withdraws from the 2014 tour, performing a last show in Mexico City on May 31. Mexican fans did a very touching "farewell show". Lewis was replaced by Sandra Laureano as touring member, but remains as member of the band for studio recordings.