Toppmoller will take charge of the club following a successful stint at Eintracht Frankfurt between 2023 and 2026.
Sage had guided the club to their first Coupe de France title and secured qualification for the UEFA Champions League only for the fourth time in Lens' history.
Lens finished their campaign last season in second place, six points behind Paris Saint-Germain, and Toppmoller will look to continue the club's progress in the coming season.
"My style of play is about finding the right balance between technical quality and intensity, and I think that will work very well with our team," Toppmoller said.
“I love intense football, and I think Lens last year was already the most intense team in the league. I love attacking football; I was already like that as a player. But the most important thing is the willingness to give everything for this shirt."