Born in Limanowa,[7] Niewiadoma started racing bikes with local club WLKS Krakus BBC Czaja and soon emerged as one of the most promising women riders in Poland with TKK Pacific Toruń.

[8] Preparing for the Giro d'Italia Femminile, Niewiadoma participated in the Polish National Championships, taking bronze in the time trial and finishing 8th in the road race.

Her climbing abilities proved to be an asset for the Rabo–Liv team at the Giro d'Italia Femminile, as she worked for the final victory of Marianne Vos on the mountain stages in the Alps.

Together with Anna van der Breggen and Vos she dominated racing during the inaugural edition of the Ladies Tour of Norway and placed 3rd,[9] winning the young rider and mountains classifications.

In September, she signed a two-year contract extension with the Rabo–Liv team and concluded preparations for the World Championships with a start in the Premondiale Giro della Toscana, placing 8th overall.

On a hilly World Championships route in Ponferrada, Spain, she led the Polish team in the road race, coming to the finish in 11th place after a sprint from the selected group.

She once again demonstrated her climbing abilities with 6th place in the inaugural edition of the Strade Bianche Donne,[8] and 5th in the prestigious World Cup race La Flèche Wallonne Féminine.

[14] Despite being only 20,[2] Niewiadoma started the season's biggest race – the Giro Rosa – as one of Rabo–Liv's leaders, alongside world champion Pauline Ferrand-Prévot and Anna van der Breggen.

[16] The long climb to San Domenico di Varzo proved decisive and Niewiadoma crossed the line in 7th, dropping to 5th overall – although she won the white jersey for best young rider and contributed towards Van der Breggen's overall success.

[20] Niewiadoma improved on her previous result at Strade Bianche – the first event to be held as part of the UCI Women's World Tour – finishing second to Lizzie Armitstead in Siena.

She was part of the leading group during the road race at the Rio Olympics, but was unable to follow moves by five other riders, and missed out on a medal; she ultimately finished in sixth place.

[36] At La Flèche Wallonne Féminine, Niewiadoma attacked inside the final 10 kilometres (6.2 miles), on the Cote de Cherave,[37] with only Deignan and Van der Breggen able to follow.

[38] Deignan forced Niewiadoma's hand with an attack before the Mur de Huy, with her Boels–Dolmans teammate Van der Breggen ultimately pulling clear prior to the final ascent.

[40] Niewiadoma's next start was at The Women's Tour, where she won the opening stage after a nearly 50-kilometre (31-mile) solo breakaway, with a winning margin of 1 minute, 42 seconds over teammate Marianne Vos.

[43] She led the WM3 Pro Cycling team at the Giro d'Italia Femminile, where she finished in sixth place overall, and ceded the World Tour lead to Van der Breggen.

[64] Niewiadoma attacked on the last ascent of the Cauberg with former teammate Marianne Vos,[65] before pulling clear with a gap of a few seconds over Annemiek van Vleuten; Van Vleuten attempted to close down the margin in the final 2 kilometres (1.2 miles) but Niewiadoma was able to hold on for her second Women's World Tour one-day victory,[66] after her earlier success at the 2018 Trofeo Alfredo Binda-Comune di Cittiglio.

Three top-ten finishes in the Ardennes classics followed, with her best result being another sprint-à-deux loss, this time to Anna van der Breggen, at La Flèche Wallonne Féminine.

[89] Further fourth-place finishes came at the Liège–Bastogne–Liège Femmes and the Durango-Durango Emakumeen Saria,[90][91] before missing out on a stage victory at the Vuelta a Burgos Feminas to Cecilie Uttrup Ludwig.

[92] After a sixth-place finish at La Course by Le Tour de France, Niewiadoma elected to forego participation in the Giro Rosa, instead concentrating on the road race at the COVID-19 pandemic-delayed Tokyo Olympics.

[107] She returned to the podium at the Itzulia Women, finishing third on the opening stage and ultimately, third overall behind SD Worx teammates Marlen Reusser and Demi Vollering.

[108][109] In the Tour de France Femmes, she finished third overall for the second year in a row, as well as winning the polka-dot jersey as winner of the Queen of the Mountains (QoM) classification.