[10] From 2014 to the show's closure in 2015, Winternitz took over the lead role of Girl from Zrinka Cvitešić in West End production of the Irish musical Once at the Phoenix Theatre,[11] first opposite David Hunter and then Ronan Keating as Guy.
[12] Winternitz made her television debut guest starring as Lisa in the Doctor Who series 9 episode "The Zygon Invasion".
Winternitz made her feature film debut with a small role in the 2016 comedy A Street Cat Named Bob.
In 2017, she starred opposite Adrian Lukis in the UK premiere of Halley Feiffer's I'm Gonna Pray For You So Hard at the Finborough Theatre,[15] for which she was once again nominated for an Off West End Award.
Winternitz returned to television when she had a recurring role as Harriet Dowling the 2019 Amazon Prime series Good Omens.