The chain kicked off the pitch in September last year amid press coverage of restaurants holding on to staff tips. The process was run by AAR.
Isobel will handle all brand and communications work for Prezzo, which most recently worked with Libertine for its on-site branding.
TPG Capital acquired the chain for £300 million at the beginning of last year. Prezzo has more than 240 restaurants across the UK after opening its first site in 2000.
Christian Poole, the marketing director at Prezzo, said: "From the start, Isobel has shown a passion for this brand that has become infectious throughout the company.
"They have strategic strength and outstanding creative credentials which should stand us in good stead to take the brand to the next level."