Though born in Westminster, Bernath grew up in Virginia, USA. She then moved to Wiltshire, England, when she was twelve years old. She 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.
In 2015 Antonia married Oliver Cockerham and became Antonia Cockerham. Her performing name remains Antonia Bernath.
Prior to studying at the Bristol Old Vic, Bernath was part of the National Youth Music Theatre and played parts including Guinevere in their production of "Pendragon". She was also the face of Johnson and Johnson, Clean and Clear facewash for an International Campaign. She debuted on British Television in 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 gained television experience portraying Priscilla Presley in the CBS television film, Elvis (2005), alongside Jonathan Rhys Meyers, Randy Quaid and Robert Patrick. Her further television roles include portraying Lucy Bellham in Heartbeat, and Nikki Brown, a semi-regular, in Holby City.
She went on to appear in the film St Trinian's in 2007, as posh totty, Chloe, and can also be seen playing the Weather Girl for Radio Sunshine, in the Richard Curtis film The Boat That Rocked. She has recently finished filming the role of Charlotte Arc in the 2009 ITV2 drama Trinity. alongside Charles Dance, Claire Skinner, and Reggie Yates.
Bernath was cast as jealous sister Jimi in British thriller Cuckoo, co-starring Richard E. Grant and Laura Fraser. Cuckoo was shot in London and directed by Richard Bracewell.
In 2012, Bernath is currently appearing in Chariots of Fire, the stage adaptation of the film of the same title. She plays Florence Mackenzie, the fiancée of Eric Liddell.
Bernath plays the female lead in the British horror comedy Stalled directed by Christian James and starring Dan Palmer.
She appears as magazine editor Laura Edmunds in the final season of Downton Abbey.
She appears as a voice actor in Witcher: Blood and Wine as Sylvia Anna.