The England midfielder and fellow Three Lions international Ben Chilwell scored two away goals in the rearranged clash in Seville to give the Blues one foot in the last four.
But Mount knows there is still work to do in next week’s return tie, which will again be held in Spain.
“We knew it was going to be a tough game. Coming into the game, we knew how hard to play against them it is and that we wouldn’t have many chances, but we kept going,” he told BT Sport.
“We obviously got the goal first half and then late on in the second, and that obviously secured a good win. But that’s only half the tie, we’ve still got the second leg to go.”
He added: “The job is not done. They are going to come and fight for the next leg and we’re going to have to give it our all again.”
Blues manager Thomas Tuchel believes the 22-year-old had elevated his career once again on a memorable night.
"The first goal was very important for us,” he added. “We had been a bit too tense, not precise enough and we had lacked a bit of rhythm.
"The finish was clinical, it was excellent from Mason. He has the quality to do this, and in a quarter-final it is a big step for him and a big, big help for the team.
"Mason has the right mentality and the right attitude towards training and games. And he has the right attitude towards success. He's got both feet on the ground, and he's open enough and hungry to learn.
"It's very difficult for me to compare him to different players in different positions, and different players in the same position from different clubs. But Mason is a very important player for us. A key player for us."