As per the BBC, the Argentina international was behind the wheel of his black Audi RS 3 when he set off a speed camera close to United's Carrington facility.
The offence occurred on August 26th last year, only four days before he signed for Chelsea after being thrown out the side by former manager Ruben Amorim.
The winger was recorded travelling at 50mph in a 40mph zone and has been slapped with a £660 fine, with an order to pay £120 in costs, a £264 victim surcharge plus three points on his license.
Garnacho pleaded guilty to speeding at Liverpool Magistrates' Court, via lawyers, and issued an apology.
"Mr Garnacho accepts that he committed the offence of exceeding the speed limit and has pleaded guilty to the offence," his lawyers at JMW Solicitors stated.
"Mr Garnacho explains this was an oversight on his part and apologises for the offence.
"Our client is embarrassed that he is currently in this position and apologises to the Court for the excess speed offence."
Garnacho has scored seven times for the Blues to date since making his £40M move and Amorim, who forced him out of the side, has since been sacked at United.