The Legal Evening News said a Beijing court convicted Sun Bing, 42, for disrupting public order by throwing ink at the portrait overlooking sprawling Tiananmen Square on March 6 last year, when the country's legislature was meeting in its annual session.
It said Sun was caught at the scene and admitted the crime in court.
The newspaper did not say what motivated Sun. People with grievances sometimes attempt to attract attention on or near the square when the legislature meets in the Great Hall of People on the square's east side.
Evidence of the ink attack was quickly cleaned up, and order was rapidly restored on the square, which is usually heavily guarded and has extra security personnel in March when the National People's Congress convenes.
The portrait of Mao, the founder of Communist China, carries great political significance because it enshrines him as a pillar of the Communist Party's rule.