The Bottom star played the role of Bombardier Bedford for three years until his death last summer, but now Mortimer will fill his big shoes.
He will star in a new ad campaign later this month, and hit the road on a promotional tour.
Phil Toms, director of brands at Charles Wells, said: “Bob Mortimer enters the fray as Bombardier Bedford following the sad loss of his predecessor and makes an instant impression.
“I’m thrilled that in Bob we have one of the country’s most original comedy talents bringing his own unique comedy to the character and we believe he’ll have a great connection with licensees and drinkers as he leads the charge for a quality pint of Bombardier.”
The campaign will be spearheaded by a 30-second TV ad, which sees Mortimer leading his troops into battle “somewhere hot and a tad dangerous”.
It will hit screens on April 23.
Marketing manager Emily Hudson promised the transition between Mayall and Mortimer will be a sympathetic one.
She said: “Our mission is to connect with drinkers through the Bombardier regiment of beers, of which there are now two variants. The hero of this campaign is Bombardier Burning Gold, the next in line in the Bombardier chain of command, and picks up from where our original Bombardier Bedford was lost in battle.
“The new character signals the next generation and will continue the irreverent and light hearted call to action that saw awareness of Bombardier rise to unprecedented levels for ale advertising in consumer tests conducted by Cardinal HPI.”
Mortimer, who collaborated with Vic Reeves in Vic and Bob’s Big Night Out and Shooting Stars, said: “I enjoyed the previous Bombardier campaign and love the fact that it symbolises great camaraderie – just like having a drink with your mates. It represents a slightly irreverent way of doing things but always with a twinkle in the eye and I’m looking forward to following the Bombardier’s unique take on events over the summer.”