Romero beat the record held for 81 years by Francisco Bao Rodríguez, known as 'Sanson', who played on December 31, 1939 aged 15 and 255 days for Celta Vigo against Sevilla.
Mallorca coach Vicente Moreno was seen talking to Romero before he brought the youngster on as a substitute.
"I tried to just keep him calm," Moreno said. "I put my hand on his heart and I could tell it was beating very fast, which is normal for a kid of his age. He deserved to play and that's the reason he came on. We don't look at age. He has been training well and this seemed the best way to increase his motivation. His a great player and I think in the future he will give us a lot."
"We saw him playing, he was doing incredible things in his age category, but physically he had not quite finished his growth and that could bring him problems if he came up against first division players too early.
"Today, when we see his feet, his body, we see that he is starting to form. And the fact that his father was a footballer also helps him a lot."
Mallorca are fighting relegation in La Liga. They sit 18th, three points away from safety with seven games left to play.