The photo, which referred to Hawthorn’s win over Sydney in the AFL Grand Final, depicted 2 Lego characters simulating sex. It was later deleted.
But media drew attention to social media criticism which suggested it was “homophobic.”
“I apologise unreservedly for the photo I tweeted yesterday – it was immature and inappropriate,” he said later in a statement on his website.
“In the excitement of celebrating the Hawks Grand Final win, I didn’t think about the homophobic implications of the image.
“This incident probably shows how entrenched inadvertent homophobia is in our culture.
“As a long-time supporter of the rights of Australia’s gay community, it’s not good enough and I’m ashamed if I have contributed to it, even in a small way.”
Mills, who appeared last week on Reality Check, has a long history of supporting the gay community, including calling for marriage equality.