He will take a stake in the company and work alongside David Billing, executive creative director; Nic Ost, chief operations officer; and Christian Lobo, executive producer. They will be roughly equal partners.
Above & Beyond, which is four years old, counts Made.com, Betway, Amazon, London Designer Outlet, Pol Roger and Abel & Cole among its clients.
Al-Zaidy left McCann in October 2015. He was promoted to chief executive from chief strategy officer in 2013.
Before McCann, he was chief strategy officer at TBWA\London. He has also worked at Mother and Rainey Kelly Campbell Roalfe/Y&R, and as a marketer at Unilever.
Ost said: "Zaid brings a wide angle view of the industry, which is essential for a future-focused agency."