She went undrafted in the 2017 WNBA Draft, and joined the Los Angeles Sparks for the 2017 season and played 3 pre-season games, but a fractured left foot suffered during training camp made the team waive her. Samuelson then joined Pallacanestro Vigarano in Italy's Serie A1 for her first professional season. Samuelson left in January, having played 13 games for Vigarano, alleging personal reasons. Afterwards, she became an assistant coach for the Vanguard Lions, while pursuing a master's degree in Coaching and Athletic Administration from Concordia University Irvine, and also re-signed with the Los Angeles Sparks for the 2018 WNBA season where she made the final roster. Samuelson made her WNBA debut on the 20th of May against the Minnesota Lynx scoring 3 points in 13 minutes of playing time, and ultimately appeared in 20 games of the 2018 season and 2 playoff games, averaging 4.2 minutes per game. While Samuelson left after four games due to the return of starter Jantel Lavender, the Sparks eventually brought her back for the rest of the season in June. Following the Sparks' elimination in the playoffs, Samuelson signed to play in BC Castors Braine of Belgium during the offseason. In Belgium, she averaged 16 points per game in the national league, and 9.7 in the Euroleague. After being waived during the Sparks training camp, Samuelson joined them late in the 2019 WNBA season, following her performance in the EuroBasket Women 2019. In August 2019, she joined the Dallas Wings. Samuelson also signed to play in Spain for Perfumerías Avenida. In 2020, Samuelson was renewed by the Wings, which also signed younger sister Katie Lou Samuelson, but wound up waived before the season begun. Katie and Karlie still wound up playing together for Avenida by the end of the year. Karlie signed again with the Sparks in 2021.