Following the 2014–15 season, Samuelson was named the national player of the year by Gatorade, USA Today, McDonald's, and the Women's Basketball Coaches Association, won the Naismith Trophy, and made the consensus All-American first team.

During the season, Samuelson scored 11.0 points per game and led the team with 78 made three-pointers, which was the second-most ever by a Connecticut freshman.

On March 6, 2017, Samuelson scored a career-high 40 points against USF; she also set the NCAA single-game record for most three-pointers made without a miss, with 10.

Samuelson was traded to the Dallas Wings for Azura Stevens on February 14, 2020, making Katie Lou and her sister Karlie teammates for the first time as professionals.

The sisters still wound up playing together in Spain as Samuelson signed with Karlie's team Perfumerías Avenida.

[9][10] Samuelson was traded on February 10, 2021, to the Seattle Storm for the number one overall pick in the 2021 WNBA draft, who became Charli Collier.

[12] Her tenure with the Storm ended on February 3, 2022, when she and the 2022 WNBA draft ninth pick were traded to the Los Angeles Sparks in exchange for Gabby Williams.

[15] On February 10, 2025, Summuelson was waived by the Fever and signed a mutual contract termination agreement with the team.

[16][17] On December 20, 2024, it was announced that Samuelson had signed with Phantom BC (filling their wildcard spot) for the inaugural 2025 season of Unrivaled, the women’s 3x3 basketball league founded by Napheesa Collier and Breanna Stewart.

[23] In February 2024, Samuelson joined the WNBA Changemakers Collective and their collaboration with VOICEINSPORT (VIS) as a mentor, "aimed at keeping girls in sport and developing diverse leaders on the court and beyond the game.