Riddle has been at the agency since 2002 and succeeds Kerry Feuerman, who resigned from the shop last week.
Riddle was previously group creative director on Travelers Insurance, Holiday Inn, and The Bahamas Ministry of Tourism. His notable recent campaigns include Travelers' "In-Synch", "Bahamavention" for Bahamas, and "Signs" for Holiday Inn.
Pat Fallon, chairman of Fallon Worldwide, said: "Todd delivered a high volume of exceptional creative work over the past year, with successful campaigns for Travelers and Bahamas, further showcasing his creative abilities and leadership potential.
"More importantly, he has a strong sense of business and is a good listener -- that's why he is so valued by his clients. Todd has my full support, and the unequivocal support of our creative department. I am excited for him."
Prior to Fallon, Riddle worked in Boston at Arnold Worldwide, and before that, Hill Holliday and Houston Effler. His client work there included John Hancock, Hyatt Hotels and Resorts, Blue Cross, Titleist, Reebok, The Lottery, The Boston Globe, and The Boston Ballet.
Riddle trained as an art director, but also writes. He has written and directed a short film and two award-winning screenplays, including Best of Show for Best Original Screenplay at The American Screenwriter's Association.
Riddle graduated from Syracuse University's School of Visual and Performing Arts and is a native of Boston.
Riddle said: "Fallon has a great legacy, a lot of great clients, and a lot of extremely talented and bright people throughout the company. It's a privilege to be part of this brand's story at any level."