The midfielder, who cost £24.4m from Lyon in 2005, has agreed a contract to promote the Irish stout in his native Ghana.
But a row could now be brewing with Mourinho, who strongly believes his players should follow healthy diets and lifestyles - and will be dismayed that his prize signing is endorsing alcohol.
Essien said: "I am very excited to be Guinness's first football ambassador, and truly feel proud to be part of the greatness that is African football.
"I hope I can act as a source of inspiration for many football fans across Africa, in the same way that Tony Yeboah and George Weah inspired me to reach greatness within my football career."
Guinness recently reported a 17% rise in sales in Africa, in contrast to slumps in the UK and Ireland.
The company's Ghana marketing head Agnes Emefah Essah said: "Guinness Ghana Breweries have a longstanding association with Ghana's Black Stars, and we believe that there is greatness in our players."
