Most recently, she was president of NBC broadcast sales and responsible for the company's network prime-time, news and sports advertising.
"I've worked with Horizon Media for many years, and have been consistently impressed with their creative mindset and personal approach to driving business value," Ms. Gambelli said in a statement.
In her new agency role, she will oversee all offline media activation strategies. She'll also be tasked with developing multi-year and multi-platform media deals, as well as seeking out new and emerging technologies and media research capabilities.
She succeeds exec VP and Chief Media Negotiation Officer Aaron Cohen. Mr. Cohen will move into a senior advisory role reporting to Horizon CEO and founder Bill Koenigsberg.
For Ms. Gambelli, it's a return to agency life. Prior to joining NBC as an account executive in 1989 and moving steadily up the ranks, she worked in various media positions at agencies such as Backer Spielvogel Bates and Grey.
"After many great years at NBCU, I'm excited to return to my agency roots, and to apply my experience across a diverse range of clients," she said.
At NBC, Ms. Gambelli was credited with helping the network improve its response rate to sponsors who had been asking for more product placements, as well as devising new creative ways of pairing product integrations on programs with more strategic media plans. She appeared on Ad Age's list of the 100 Most Influential Women in Advertising last September, with buyers saying they respected her plainspoken attitude and contributions to innovation at NBC.