Shepherd joins the agency from OgilvyOne UK, where he was associate director of mobile for a year.
He replaces Dan Beasley, who left Jam last year to set up his own agency.
Shepherd will be responsible for leading Jam's global mobile strategy across clients and expanding the agency’s mobile capabilities.
He will report to deputy managing director, Alice Driscoll.
Prior to OgilvyOne, Shepherd worked at mobile business Blippar and M&C Saatchi Mobile.
Shepherd’s appointment comes amid a raft of new hires at Jam, including associate creative director, Iskra Tsaneva and operations director, Harriet Playford.
Tsaneva will work across clients such as Xbox, Renault and Sky and help oversee the agency’s 35-strong creative team, reporting into Jam’s executive creative director, Sean Chambers.
Her appointment reunites her with Chambers, who she worked with at Razorfish, when she was a senior creative. She has also worked at Mother, Leo Burnett and Wieden & Kennedy Amsterdam.
As head of operations, Playford’s remit at Jam will be to develop its structure, process and delivery practices in line with its broader agenda.
She was most recently at Dare where she worked for eight years as operations director. She was appointed the agency’s board in 2009 and was also part of the agency’s Bristol production office management team.
Richard Costa-D’Sa, managing director at Jam, said: "James will be key to growing our mobile strategy globally, while Iskra and Harriet play a pivotal role in broadening our creative and operational output. We welcome them to the Jam family."