Mandlik made her WTA Tour main-draw debut at the 2019 Luxembourg Open in the doubles tournament, partnering with Katie Volynets.

240, Mandlik qualified for the main draw and earned her first WTA Tour tournament win at the Silicon Valley Classic, defeating Alison Riske-Amritraj in straight sets.

[6] She made her debut at the Australian Open as a lucky loser, but was defeated in the first round by 27th seed Irina-Camelia Begu.

[7] At the Nottingham Open, she qualified for the main draw and defeated Viktoriya Tomova[8] and seventh seed Camila Giorgi to reach the quarterfinals,[9] where she lost to Alize Cornet.

[11] Mandlik is the daughter of Grand Slam champion Hana Mandlíková[12] and granddaughter of an Olympic runner, Vilém Mandlík.