He picked up a friend and was caught filling the tank with petrol at a garage.
Mallen, right, was given a three-month suspended sentence after he paid €1,000 in compensation to the taxi driver. His friend, Louis Parsons (20), was accused of being a passenger in a stolen car and had his case adjourned for a probation report.
Mallen, of Alpine Heights, Clondalkin, pleaded guilty to taking a vehicle at Melrose Lawns, Clondalkin on February 29.
A charge of producing a knife was withdrawn by the prosecution.
Parsons, of Melrose Lawns, admitted being a passenger in a stolen car at Esso garage, Naas Road.
The prosecuting garda said he was on patrol at 5.20am when he saw a red Toyota in the garage and the defendants were standing beside it.
The court heard had been so intoxicated on arrest that a doctor had to be called to the garda station and declared him unfit for interview for six hours.