She previously served as press secretary to Cameron while he served as Conservative Party leader, a role which she retained when he was elected as prime minister. While on maternity leave in mid-November 2012, she was replaced by Susie Squire. Upon her return to Downing Street in August 2013 , Bertin was appointed as director of external relations.  In this role, she was be 'responsible for forging – and maintaining Downing Street’s – relations with business, pressure groups and charities.' 
Bertin was a well trusted and long standing adviser, according to PRWeek, which cited one source recently claiming the ‘hole she will create is impossible to fill'.
From the House of Commons register of interests in 2007:
- 'I work exclusively for the Atlantic Bridge, a UK-American think-tank of which my sponsor, Dr Liam Fox, is a founder member. In this role I receive funding from Pfizer Inc. I have no function in any health role (Dr Fox is Shadow Secretary of State for Health). I also do occasional work from home for Parker, Poe, Adams & Bernstein (an American law firm based in North Carolina).' 
- Membership and principal office holders, Parliament.uk, accessed 3 February 2017.
- Resignation honours 2016, GOV.uk, accessed 3 February 2017.
- Alice Thomson 'The Camp David team' Daily Telegraph, Filed: 07/12/2005.
- John Owens New roles for Bertin and Wilson, PR Week, 27 August 2013, accessed 8 September 2014
- George Eaton David Cameron appoints the Sun's deputy political editor as his press secretary News Statesman, 27 August 2013, accessed 15 October 2014
- James Forsyth Change at Number 10 The Spectator, 31 October 2012, accessed 10 October 2014
- Parliamentary Publications & Records Register of Interests of Members' Secretaries and Research Assistants last accessed 31st May 2007