They'll oversee planning and account-service teams to help Horizon meet the needs of its clients, which include Geico, the History Channel, the Meow Mix Co. and WWE.
Horizon has split its client list into three sectors, and Ms. Kantrowitz, Ms. Pollock and Oliver Maletz, who joined the company a year ago, each will manage a set of accounts.
"Initially we had two managing directors," said Gabrielle Magnani, who just moved from managing director to exec VP-marketing and brand management this month. "We assessed our client needs. As we are growing, we want to manage for current growth and manage for future growth."
Ms. Kantrowitz joined the company in January. "She is able to translate marketing and media and bring those disciplines to meet our clients' challenges," Ms. Magnani said.
Ms. Kantrowitz, who oversees Geico, the History Channel and David's Bridal, said, "I was very attracted to Horizon because they are so vital. There's a lot of entrepreneurial attitude going on, and I'm very happy it's an independent."
Ms. Pollock is a 10-year company veteran. "I've really seen the evolution," she said. "I feel I've been part of this evolution. Horizon has really grown in terms of strategic planning, and we have been very successful with that."
Changes within Horizon are just the beginning, Ms. Magnani said. "As new clients enter the fold, we simply have more opportunity to apply our media expertise and develop new solutions across more categories," she said. "Evolving our services is necessary in such a dynamic business."