[1] After finishing drama school, Kempner began his career as an actor, portraying minor roles in the films Fathers of Girls (2009) and Fly Trap (2010) and appearing in musicals including Lift, Les Misérables, South Pacific and Avenue Q but said that he "always wanted to do [his] comedy professionally.
"[1][5][6] In 2012, he began posting videos of himself impersonating celebrities including Andy Murray and Tom Daley, however it wasn't until his comedy skit Downstairs at Downton, inspired by the characters of the ITV drama Downton Abbey, gained traction on social media and his career as a comedian and impressionist subsequently took off.
[7] In 2016, he appeared in an episode of Drunk History and provided the voices for the ITV sketch show Newzoids, before joining Big Brother's Bit on the Side as a regular comedian and impressionsist, in which he impersonated the housemates throughout the series.
[16] Between 2020 and 2021, he provided the voices for the puppets on the revival series of Spitting Image and went on to star in the musical adaption.
[19] In 2023, Kempner impersonated various celebrities in the ITV2 comedy series Deep Fake Neighbour Wars, before it was announced he would be appearing in the West End production of Les Misérables as Thénardier.