Judge Roger Chapple, at Inner London Crown Court, told Umaniec: "Your actions on the 7th of October of this year were entirely deliberate, planned and intentional."
He said it was "abundantly clear" that Umaniec was "plainly an intelligent man" and told the court he had described Rothko as a "great painter" in a letter he had written to him.
The judge also said the incident had led to galleries reviewing security arrangements at a cost to themselves and the taxpayer.
"The effects of such security reviews is to distance the public from the works of art they come to enjoy," he said.
More top news
Hospital chiefs asked by NHS regulator to fill vacancies only where "essential" as they face an unprecedented financial challenge.
The footballer and father-of-two said he is 'filled with pride' at being asked to take on the role.
A state of emergency has been declared as emergency crews struggle to overcome raging wildfires that have been burning for six days.