Around 20 protesters "overwhelmed" security at the fast food chain’s outlet in Brighton on May 18, holding signs and chanting through a megaphone.
Actress Dylan Roffey, who was pictured covered in the fake blood and wearing a pig mask, claimed in court on Thursday that her actions were legal because she was trying to save animals from slaughter.
The 24-year-old said the red substance was a "completely edible" mixture of flour and food dye, designed to highlight what she called animals being "murdered".
However, a judge at Brighton Magistates’ Court rejected her defence and found her guilty of criminal damage.
Roffey, of Terminus Road in Brighton, appeared in court in a t-shirt with the words "meat the victims" written on it.
She was cleared of obstructing a police officer, with District Judge Amanda Kelly saying she was "not sure" the PC’s force was "reasonable".
McDonald’s assistant manager Robert Frost, who was in charge of the restaurant on the night, told the court that between 10 and 20 protesters had arrived at about 7.30pm.
"They overwhelmed security," he said.
"One of them had a loudspeaker chanting slogans about how there is animals being killed and they began throwing red paint, to show blood, across the premises."
He added that "some of the customers were quite upset."
Mr Frost told the court he called the police and many of the protesters started to leave, but one woman refused.
He said he saw officers having to pull her up and take her outside.
Earlier, Roffey’s solicitor Meredoc McMinn sought to have the resisting a police officer charge dismissed on the basis of "unreasonable force" but Judge Kelly declined to do so and the case continued.
Roffey told the court her motivation was: "To raise awareness of what McDonald’s is doing to animals and to save the lives of animals by being there and stopping the sale of their bodies.
"By raising awareness through what we were saying, changing people’s minds and secondly every sale that they do not make results in less animals being murdered."
Mr McMinn argued that Roffey "made a point of spreading (the fake blood) on surfaces that would normally have been cleaned".
In sentencing Roffey, Judge Kelly said: "Not withstanding the fact that the mixture was flour, water and food dye… the damage need not be permanent in order to be criminal.
"I am absolutely sure that Miss Roffey intended that damage.
"I find that Miss Roffey’s purposes was to raise awareness and attract publicity for her cause but that these purposes are too far removed from providing the animals’ immediate protection.
"I have a lot of respect for a young woman with strong principles which you clearly do but this is not the way to go about it."
The judge also noted customers who are "legally entitled to go to McDonald’s for their Friday night dinner" had to leave because of the process.
Roffey - who last week was sentenced to 150 hours of unpaid work for an unrelated assault - had another 60 hours of unpaid work added to her sentence.
She must also pay £50 compensation to McDonald’s, costs of £250 and a victim surcharge of £85.