A mix of a Norwegian father and a Brazilian mother, he took part in his first competitions at the age of 16. The following season, he won several slalom and giant slalom races and took part in his first Junior World Championships in Davos, where he finished sixth in the slalom.

He finished second in the European Cup giant slalom and had three European Cup giant slalom victories prior to the Junior World Championships, where he won silver in the super-G and bronze in the combined.

He won his first World Cup race in October 2020 at the finish of the giant slalom in Sölden. At 20 years and 182 days, he is the youngest winner of an event on the international circuit since Henrik Kristoffersen in the 2014 Levi slalom. In January 2021, he fell heavily and tore ligaments in his right knee, forcing him to interrupt his season prematurely.

One year later, in January 2022, he won the second race of his career with an incredible feat in the Wengen slalom: 29th in the first run, he achieved a great performance in the second run. Lucas Braathen is the first racer in World Cup history to have climbed 28 places in a technical event.

During the 2022 World Cup finals in Courchevel, he finished 2nd in the giant slalom, a promising performance for the 2023 World Championships!