[5] Käslin made her senior debut at the 2013 European Championships in Moscow, Russia, qualifying 18th into the all-around with a 51.499 and improving her final score to 53.632 to finish thirteenth overall.

[18] At the World Challenge Cup in Doha in March, she placed seventh on bars and won the bronze medal in the floor final with a score of 13.750.

[21] Käslin competed at the World Championships in Glasgow, where she posted an all-around score of 53.456 towards the Swiss team's 16th-place finish, ensuring them a spot at the Olympic test event in April 2016.

[22] Directly after the World Championships, Käslin headed to the Arthur Gander Memorial where she placed 5th,[23] then went straight to compete at the Swiss Cup where she and Pablo Braegger teamed up to finish 4th.

In 2016, Ilaria participated with the Swiss team in the Test events in Rio de Janeiro, the final qualifying stage for the Olympic Games.

Käslin was named to the team to compete at the 2019 World Championships in Stuttgart, Germany alongside Giulia Steingruber, Stenfanie Siegenthaler, Anny Wu, and Caterina Barloggio.

She holds a Bachelor's degree in Psychology and is currently pursuing a Certificate of Advanced Studies (CAS) at the University of Lausanne in Mental Preparation for Athletes, Musicians, and Dancers.