Steve Paskin, Now’s previous executive creative director, left last year.
Graham will become a partner alongside the chairman, John Townshend, the managing partner, Melissa Robertson, and the strategy partner, Kate Waters.
Townshend was previously the agency’s creative partner but became the chairman at the end of 2015.
At Mcgarrybowen, Graham partnered with Richard Holmes. The pair won a gold Lion at Cannes last year for Honda "illusions" and a silver for Weedol "still weeding".
Before joining Mcgarrybowen in 2012, Graham worked at Mullen Lowe London (then known as DLKW Lowe) for eight years. His career began at WCRS in 1997 and includes a stint at HHCL.
Graham said: "What a great brief. Make an agency famous and share in the running of the business and its success. Now is one of the most interesting start-ups of recent years and has a fantastic team of partners."
Meanwhile, Mcgarrybowen has hired Stephen Cross and Simon Connor as creative directors. They join from Karmarama, where they have worked since 2009, handling accounts including the BBC and Betfair.