Jay Makwana, 40, of Northumberland Park, Erith pleaded guilty to robbery at Woolwich Crown Court on Wednesday, 6 January.
Police were called at 10:52am on Thursday, 9 November to reports of an injured woman in Market Place.
Officers attended and found an 81-year-old woman lying on the ground with facial injuries. She was taken by London Ambulance Service to a south London hospital where her injuries were assessed as not life-threatening.
Enquiries were carried out and it was established that a purse containing approximately £200 was stolen from the victim.
Officers launched an investigation taking witness statements and reviewing CCTV from the area.
The footage showed the victim leaving a nearby bank when she was approached by Makwana. He pushed her to the ground causing her to hit her head on the pavement.
He waited with the woman until officers arrived before putting her purse into a carrier bag and leaving the scene.
Makwana was arrested by officers from the South East Basic Command Unit on 24 November. He was interviewed and charged the following day.
Sergeant Matt Taylor, of the Burglary Robbery Investigation Team, said: “This was a truly shocking assault on a woman who was deliberately targeted because of her age and vulnerability.
“It has had a huge impact on the victim’s life and left her scared to leave home alone. I hope the fact that Makwana received a significant custodial sentence will bring her some closure as she continues to come to terms with this terrible ordeal.”
Makwana was also ordered to pay a £190 victim surcharge