Warner will become her main aide on comms, as well as providing policy advice, when he assumes the post at the end of October.
He said the role's biggest challenges will be around delivery of the London 2012 Olympics - such as encouraging more take-up of sport - digital switchover and upholding the importance of the creative industries.
Warner, who will join existing special adviser Nick Bent, joined Lexington in 2001. For the previous four years he was special adviser to Mo Mowlam.
He was Mowlam's aide during her time as Northern Ireland Secretary and her spell at the Cabinet Office.
Warner's clients at Lexington have included ITV and Royal Society of Arts.
‘I'll miss Lexington and have thoroughly enjoyed being part of its success over the past five years, but some opportunities are too good to pass up,' said Warner.
‘This is a fascinating time in British politics with the challenges ahead for the Department of Culture, Media and Sport and the Labour government,' he added.
Lexington founding partner Ian Kennedy said it would look to replace Warner in ‘due course', adding there was no ‘urgent rush', with the existing team covering his responsibilities.
Warner's appointment by Jowell comes after the exit of two special advisers earlier this year. Roger Sharp quit to become group director of public affairs at O2. The news was broken by PRWeek as Jowell's lawyer husband David Mills was alleged to have used a £350,000 bribe from then Italian prime minister Silvio Berlusconi to pay off the couple's mortgage (PRWeek, 4 March).
This summer, former Freud Communications director of PA Sara Latham left the DCMS to start a PA and crisis comms consultancy (PRWeek, 7 July).