She is assured of at least a bronze medal in the ongoing Women's World Championships.
The veteran used all her experience to sway away from challenges and land punches at the right time to put pressure on Valencia.
She was adjudged winner on points after a scoreline of 29-28, 30-27, 29-28, 30-27, 29-28.
She will next face second seed Busenaz Cakiroglu of Turkey.
Mary Kom has enjoyed a stellar career
Mary Kom has enjoyed a stellar career in boxing.
She has bagged six Asian Championships medals till now including five golds.
Mary also won a gold at the Commonwealth Games in 2018, besides two Asian Games medals (2010 and 2014).
The 2012 London Olympics bronze medalist has also bagged a gold in the Asian Indoor Games in 2009.
I am very happy to have secured a medal: Mary
"I am very happy to have secured a medal but I would be looking to make it better by reaching the finals," she said. "It was a good bout for me and I will now try to improve on this performance in the semis."
Mary Kom holds these superb records
She is the only Indian woman boxer to have qualified for the London Olympics and then going on to win a medal.
She became the first Indian woman boxer to win a gold medal at the Asian Games in 2014 and the 2018 Commonwealth Games.
She is the only boxer to clinch Asian Amateur Boxing Championships a record five times.