The message came just days after The Sun, which she edited, deserted Gordon Brown to back the Conservatives before the 2010 General Election.
But it was also a sign of the close relationship between senior politicians in the Tory party and executives at News International.
The eight month-long hacking trial has been a difficult one for Downing Street.
The longer it dragged on the closer the fallout from its result — and the redredging of questions about David Cameron's relationship with Mrs Brooks and his former spin doctor and ex-News of the World editor Andy Coulson has got to next year's General Election. Questions were raised about Mr Coulson's suitability to work for the future Prime Minister from the off.
Critics pointed to the fact that he had resigned from the News of the World in the wake of the phone hacking scandal.
But supporters cautioned that although he had fallen on his sword, because the scandal happened on his watch, there was no suggestion that he had known what was going on.
Mr Cameron was supportive.
He publicly defended Mr Coulson on a number of occasions.
And both he and the Chancellor George Osborne, who recommended Mr Coulson for the job, insist they sought and were given reassurances over hacking.
Once, when put under pressure about his communications chief, the Prime Minister famously said that he believed in giving people "second chances".
He also denied that he had hired the former tabloid editor because of his closeness to Coulson's former boss, media mogul Rupert Murdoch, pointing to his abilities to translate policy and ambitions into stories that would catch the imagination of voters.
But during the Leveson inquiry Mr Cameron also admitted that his decision to hire controversial spin doctor Andy Coulson "haunted him". And he said that if it emerged that he been lied to about phone hacking, then it would be time for him to make a "profound apology".
What was not known at the time, and which only emerged later during their trial, was that as well as being work colleagues and close friends, Mrs Brooks and Mr Coulson had also been having an affair, on and off for a number of years.
Mr Cameron's closeness to Mrs Brooks was legendary.
In messages revealed during the Leveson Inquiry into press ethics, ordered in the wake of the phone-hacking scandal, a picture emerged of dinners together with the group whose second homes neighboured each other in the Cotswolds, known as the "Chipping Norton set".
There was mention of "country supper" and a message from Mrs Brooks in which she told the Prime Minister that she was a "proud friend" of his.
She ended that particular message by paraphrasing Barack Obama's famous campaign slogan, writing "Speech of your life? Yes he Cam!"
The Prime Minister had also ridden Mrs Brooks' horse, Raisa.