Barta was the second youngest rider in the 2016 Tour of California. In August 2019, he was named in the startlist for the 2019 Vuelta a España,[6] his first Grand Tour. He also rode the race in 2020, finishing 22nd overall. He recorded a second-place finish on stage 13, an individual time trial; he had held the best time until Primož Roglič, the penultimate rider to start, bettered his time by one second.[7][8] Barta's first professional win came in the fifth and final stage of the 2024 Volta a la Comunitat Valenciana, when Barta was part of a breakaway group and then made a solo attack on the steepest slopes of La Frontera climb, riding solo for over 50 km to take the stage victory.[9]
^"UCI approves Axeon Pro Continental status". Cyclingnews.com. Immediate Media Company. December 4, 2017. Retrieved January 24, 2018. The team has announced eight riders so far for the 2018 roster, including João Almeida from Portugal, William Barta from the US, U23 time trial world champion Mikkel Bjerg from Denmark, Cole Davis from the US, Zeke Mostov from the US, Jasper Philipsen from Belgium, Thomas Revard from the US and Maikel Zijlaard from the Netherlands.
