He will be responsible for developing revenue streams at SeeSaw, owned by transmission outfit Arqiva, and which is due to launch in the next few months. Rennie will also hire a new commercial team.
Previously, Rennie worked in a business development role at Channel 4, where he led content distribution, managing commercial relations with distribution partners such as YouTube and Virgin Media.
Before his role at Channel 4, he held senior commercial roles at Virgin Media and at Warner Music Group, where he spearheaded the launch of iTunes in Europe on behalf of WMG.
He joins a senior SeeSaw management team that includes chief executive Pierre-Jean Sebert and platform controller John Keeling, who was appointed in September.
SeeSaw was born out of the remnants of Project Kangaroo, which was blocked from launch by the Competition Commission earlier this year.
Last month, Arqiva appointed Vizeum to handle the estimated £5m media planning and buying account for SeeSaw.