Mount’s slick turn and shot in the first half would have been painful enough for Porto, who were the better side for large spells in Seville but repeatedly lacked the precision to take advantage.
Instead, their misery was compounded by Ben Chilwell capitalising on a mistake at the back to score a late second that gave Chelsea another away goal and leaves them on the brink of the semi-finals.
They were away goals only in name given both these teams were playing in the unfamiliar Ramon Sanchez Pizjuan, where they will meet again next week, just with the home and away roles reversed.
Porto were in the home dug-out, wore their home kit, had a large club banner in the empty stand and their anthem was the one that played out before kick-off.
Chelsea played the part too, delivering a classic away performance of soaking up the pressure and hitting their opponents on the counter-attack, their quality in the final third proving decisive.
Real Madrid could lie in wait for the victors, after they beat Liverpool 3-1 on Tuesday.
“There is a good atmosphere in the dressing room, we are happy but not too happy, excited but not too excited,” said Chelsea’s coach Thomas Tuchel. “We have to have a top mentality and a top performance next Tuesday.”
Mount’s goal makes him Chelsea’s youngest ever scorer in the Champions League at 22 years and 87 days old while Chilwell’s second meant the club boasted two English scorers in a Champions League game for the first time since 2012.
Chelsea’s win also goes some way to easing the shock of Saturday’s 5-2 loss at home to West Brom in the Premier League, which ended Tuchel’s 14-match unbeaten start as coach.
“After losing such a strange game we said the best situation would be to produce a clean sheet straight away and this makes it very precious, this result,” Tuchel said.
– Chilwell’s sucker-punch –
Tuchel decided Thiago Silva’s 29 minutes before being sent off in that game were not enough to hone his sharpness, meaning Andreas Christensen was picked in the back three.
And Antonio Rudiger also returned, despite his training ground scuffle with Kepa Arrizabalaga on Sunday, which Tuchel described as a “serious situation”.
Porto were regarded as the plum draw, despite seeing off Cristiano Ronaldo and Juventus in the last 16, their task made harder this time by the suspensions of Mehdi Taremi and top scorer Sergio Oliveira.
But they were the aggressors from the outset, with Chelsea surrendering the ball and the chances.
Porto had six shots to Chelsea’s one before half-time, with Mateus Uribe grazing the top of the net with an early volley and the excellent Otavio almost catching Edouard Mendy out with a corner, Zaidu Sanusi smashing over the rebound.
Chelsea, though, showed them what ruthlessness looked like when Jorginho fired a pass into Mount on the edge of the area inside the right channel. Mount took it on his back foot and twisted away from the sliding Zaidu, before firing a cool finish into the far corner.
Porto kept coming in the second half and kept missing too. Moussa Marega snuck in but his shot was saved by the feet of Mendy before Jesus Corona feathered Luis Diaz through but he took too long. Diaz curled wide after a good run by Wilson Manafa.
Chelsea absorbed them and then hit them with a final sucker-punch, Mateo Kovacic’s ball over the top miscontrolled by Corona, allowing Chilwell to steam clear, round the goalkeeper and finish.