The 21-year-old winger was behind the wheel of his black Audi RS 3 on August 26 last year when he triggered a speed camera on Carrington Lane.
Garnacho was driving at 50mph in a 40mph zone just four days before he signed for Chelsea in a £40 million deal.
Through lawyers, Garnacho pleaded guilty to speeding at Liverpool Magistrates’ Court, and issued an apology saying he is “embarrassed” by the incident.
The player's lawyers at JMW Solicitors wrote: “Mr Garnacho accepts that he committed the offence of exceeding the speed limit and has pleaded guilty to the offence.
“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 was also charged with failing to respond to a Greater Manchester Police letter about the speeding offence, when he was asked to identify the driver of his car in October last year.
However, that offence was withdrawn after Garnacho pleaded guilty to speeding.
Magistrate Jane Haynes sentenced the footballer to a £660 fine, with an order to pay £120 in costs and a £264 victim surcharge.
Garnacho was also issued with three penalty points on his licence.
Garnacho joined Chelsea for £40m last summer
The Argentine international came through the Manchester United academy but left the club after he fell out of favour with former head coach Ruben Amorim.
He has made 36 appearances since switching to Chelsea, scoring seven goals.
The police computer system recorded that someone logged in to see the evidence in the case after Garnacho had been twice written to about the speeding incident.
However the force said he did not respond to the letters to confirm that he had been at the wheel of the car, which has a starting price of more than £61,000.
Garnacho was convicted and sentenced in a hearing conducted last week behind closed doors under the Single Justice Procedure.