Raised in Wiltshire, England, Bernath attended Godolphin School before going on to the University of Cambridge. She later dropped out of university when she gained entrance to the Bristol Old Vic Theatre School.

Bernath debuted on British television in 2003 on the Hogmanay special episode of Monarch of the Glen as Becky Body. Her feature film debut was in Kisna: The Warrior Poet (2005), playing Catherine, as one of the first leading British actresses in a Bollywood film. Since her debut, she has gained television experience as Priscilla Presley in the CBS television film, Elvis (2005), Lucy Bellham in Heartbeat, and Nikki Brown in Holby City. She went on to appear in the film St Trinian's in 2007, as posh totty, Chloe. She has recently finished filming the role of Charlotte Arc in the 2009 ITV2 Drama Trinity.

Antonia was cast as jealous sister Jimi in upcoming British thriller Cuckoo, co-starring Richard E. Grant and Laura Fraser. Cuckoo was shot in London in December 2007 and January 2008, directed by Richard Bracewell.