Police are appealing for witnesses after the incident on Saturday, November 19, in Weston Favell village.
A woman in a wheelchair was travelling down the alleyway between Wellingborough Road and Rowlanson Close, when a young man with a knife approached her on a bicycle.
He tried to pull her phone from her hand and cut her shopping bag from her wheelchair before making off.
The offender was described as white, between 16 and 20 years old and 5ft 9 inches tall.
He was wearing a blue and grey chequered scarf, a blue, thin, knitted beanie hat, a waist-length blue or black jacket and dark trousers.
Anyone with information can contact Northamptonshire Police on 101 or Crimestoppers anonymously on 0800 555 111.