The artwork, created by local artist AJA Louden, was painted on Friday on a public wall in Edmonton, Alberta.
A Canadian Broadcasting Corporation (CBC) journalist was shooting footage of the piece on Sunday morning when a man walked up with spray paint and began writing “Stop the Lies. This is Oil Country!!!” over the 16-year-old Swedish climate activist’s face.
He told the journalist: "This is Alberta. This is oil country. My father has worked in the oil industry. We don't need foreigners coming in and telling us how to run our business, support our families, put food on our tables.”
A mural of climate activist Greta Thunberg appeared in #yeg after she rallied in Edmonton on Friday
A CBC journalist was filming when two people began spray-painting over the teen's face.
Read more: https://t.co/6NL5N37DQb #yeg #oilsands #yyc pic.twitter.com/ifS926TMuy— CBC Edmonton (@CBCEdmonton)October 21, 2019
He added that as soon as he saw photos of the mural on social media he decided to “deal with it”.
He also said Greta is a child who is “doing what she’s told” and does not know any better.
When CBC returned later the same day to shoot more footage, a different man was again found to be defacing the mural, writing a derogatory term describing the teen.
Mr Louden posted a photo of the mural on his Instagram and responded to comments about it being defaced saying: “I’m not upset, it’s a free wall so nothing lasts forever”.
Ms Thunberg was in Edmonton on Friday to attend a climate march and rally attended by thousands of people.