Renee Weaver, 36, targeted Gerard Stevenson, 64, who uses two walking sticks, at a Croxteth newsagents on November 3, 2015.
But she was arrested again when police were called to reports of two women arguing in Belmont Road, Anfield on September 18.
Liverpool Crown Court heard both women alleged the other had a knife and Weaver was searched by female officers in a police vehicle.
Afterwards a blue-handled kitchen knife, with a three-inch blade, was found on the floor and she was transferred to a van with a cage.
Ken Grant, prosecuting, said she then spat in the mouth, face and over the clothes of shocked PC Anthony Lorenzelli.
The court heard PC Lorenzelli had to go to hospital for tests and is still awaiting the results eight months later.
Weaver admitted possession of cannabis but denied possessing an article with a blade or point and common assault.
She claimed the knife was not hers and that she had just been clearing her throat when she accidentally spat on PC Lorenzelli.
However, she was found guilty following a four-day trial, a verdict which Desmond Lennon, defending, said she was 鈥渦nable to accept鈥?
He told the court she was disappointed by the outcome and added: 鈥淪he still maintains her innocence.?/p>
The mum has multiple previous convictions, including a sneak theft from an elderly victim, plus robberies in 2003 and 2012.
She has been sentenced for possessing drugs, assault, affray, handling stolen goods, false imprisonment, burglary and possessing a knife.
Mr Lennon urged the court to impose another suspended sentence, so Weaver could seek help for her drug addiction.
He said: 鈥淪he is very much addicted and dependant upon crack cocaine and heroin. It has caused chaos in her life.?/p>
Judge Alan Conrad, QC, said Weaver would have been 鈥渨arned in clear terms鈥?about the consequences of breaching her suspended sentence for the 鈥渘asty鈥?theft.
He said the courts had also made it clear that carrying knives is a serious offence, which almost inevitably results in a custodial term.
The judge said: 鈥淵ou spat all over Constable Lorenzelli, an officer carrying out his public duty.
鈥淪pit went over his face, his uniform and even his mouth, causing him great anxiety, resulting in hospital tests that still haven鈥檛 been concluded.
鈥淚t must be noted you have been given opportunities in the past, which have tried to help you, and you have not taken these opportunities.?/p>
Judge Conrad activated Weaver鈥檚 suspended sentence and gave her a total of two years and nine months behind bars.