The former England captain previously pleaded guilty to using the device while driving his Bentley in central London on 21 November last year.
A court heard he was photographed by a member of the public holding a phone as he drove in "slowly moving" traffic.
Beckham, 44, received six points on his licence to add to the six he already had for previous speeding matters.
He was also fined £750, ordered to pay £100 in prosecution costs and a £75 surcharge fee within seven days.
District Judge Catherine Moore said she acknowledged the slow pace of the traffic but told him there was "no excuse" under the law.
Bromley Magistrates' Court heard Beckham was seen "operating a handheld device at knee level" while driving along Great Portland Street in the West End.
Prosecutor Matthew Spratt said: "Instead of looking straight forward, paying attention to the road he appeared to be looking at his lap."