Both teams were level on points following their first two games of the campaign but the Swiss side were one place above the hosts due to their superior goal difference.
Read how all the action unfolded in our commentary below.
8pmGood evening and welcome to the live commentary of the Europa League clash between Valencia and St Gallen.
8.03pmVALENCIA: Guaita, Costa, Pereira, Ruiz, Parejo, Romeu, Barnat, Cartabia, Piatti, Canales, Alcacer
8.05pmST GALLEN : Lopar, Besle, Mutsch, Montandon, Mathys, Nater, Vitkieviez, Lenjani, Rodriguez, Janjatovic, Karanovic
1 minWe're underway in this Europa League clash.
2 minCHANCE! The home side threaten immediately as the ball is played to Canales on the edge of the penalty area on the right. He cuts inside on his left foot and drills an effort towards the bottom corner but Lopar makes the save.
4 minCHANCE! Valencia go close again but are denied by Lopar as they launch the ball into the box and Besle clears but only as far as Parejo on the edge of the area and his low volley is heading towards the bottom corner before Lopar sticks out a hand to tip it away.
6 minThe home side keep up the pressure and it's all hands to the pump for the visitors at the moment as Besle has to block a powerful shot from Piatti.
8 minMutschh brings down Piatti on the left and the referee awards a free kick. Canales whips in a dangerous cross which evades everyone in the area and is claimed by Lopar.
10 minCHANCE! Now it's Gallen's turn to threaten and Guaita is forced into action for the first time to make a smart stop from an effort from Rodriguez, which was hit with power.
11 minSHOT! Canales tests Lopar again from distance and once again the keeper is up to the task as he holds the ball with ease from the left-footed strike.
12 minGOAL! Valencia 1-0 St Gallen (Alcacer)
12 minValencia take the lead and it's Alcacer who has got the goal as he pounced on the rebound after Piatti raced forward and blasted an effort at Lopar, which he couldn't hold and the striker was there to fire home the second attempt from close range.
14 minThat goal is no more than the home side deserve after their dominance in the early stages of this game as they peppered Lopar's net with attempts. One of them was bound to breakthrough eventually.
18 minThe home side seem to be taking the pace out of the game at the moment to quell the threat of equaliser from the visitors, who look slightly shell shocked.
20 minGOAL! Valencia 2-0 St Gallen (Cartabia)
20 minValencia doubled their lead in slightly fortuitous circumstances a Cartabia raids forward with the ball and beats the challenge of Rodriguez and strikes a 30-yard effort, which takes a deflection off Montandon which wrong foots Lopar and finds the bottom corner.
22 minAlcacer brings down Janjatovic allowing Rodriguez to whip the ball into the box for a set piece for the visitors but his delivery is poor and the home side clear with ease.
24 minMutsch returns the favour for the hosts as he brings down Piatti on the left. Parejo lifts the ball into the box but Besle gets up highest in the penalty area to head it clear. The visitors can't afford to concede again.
27 minSHOT! Valencia have had success with long shots this evening so Parejo tries his luck from distance but Lopar makes a comfortable save.
30 minGOAL! Valencia 3-0 St Gallen (Cartabia)
30 minOH DEAR! Lopar has had a shocker on this one as he lets Cartabia's tame left-footed shot from the right creep underneath his body as he gets himself in a tangle on the goalline and this game is getting out of hand quickly.
32 minGOAL! Valencia 4-0 St Gallen (Ricardo Costa)
32 minIt gets worse for the visitors as they concede their fourth goal as Parejo raids down the left and whips a dangerous cross to the back post, where he finds Victor Ruiz, he heads the ball back across goal for Ricardo Costa, who heads the ball into net from close range.
34 minIf there's any comfort for the visitors, that goal wasn't the fault of Lopar.
36 minCHANCE! St Gallen come close to getting a goal back as the ball bounces in the area and is played back to Lenjani, who dribble his way into the box before rifling a left-footed shot at goal, which Guaita does well to parry past the post.
37 minRodriguez delivers the ball from the resulting corner but his cross is poor and Valencia clear the danger with ease.
39 minSHOT! Another shot from distance from the home the side as they are testing Lopar's confident at the moment but this time Piatti's effort is well wide of the target.
41 minValencia work the ball down the right with Cartabia and Piatti, the winger finds the run of Alcacer in a central position 30 yards from goal and he fires a shot at goal, which goes just over the bar. They're making their intentions clear here.
44 minCHANCE! Parejo whips a dangerous corner to the back post where he finds the run of Ricardo Costa, who heads at goal but his effort is blocked on the line by Nater.
45 minHALF-TIME: Valencia 4-0 St Gallen
8.53pmWhat a first half from Valencia. They well and truly dominated St Gallen and quickly put them to the sword with four well taken goals, which were aided by some poor keeping from Lopar.
8.55pmSt Gallen couldn't cope with how quickly the hosts came out of the blocks at them and had no answer when they went ahead. They have little hope of getting back into the game and maybe struggling to keep the score respectable.
9.05pmThe players are making their way back onto the pitch and we should be back underway shortly.
46 minWe're back underway at the Mestalla.
48 minValencia have come back out for the second half in the same rhythm as they had in the first and it's hard not to see them adding to their four goals.
49 minSHOT! Valencia work the ball down the left and Piatti beats his man and cuts inside on his right foot but his shot goes wide of the mark.
54 minSHOTS! Canalaes and Alcacer try their luck from distance now but neither can hit the target. That mistake from Lopar in the first half seems to have given a signal of shoot on sight to the home side.
56 minMutsch gives away what seems to be his umpteenth tackle on Piatti. The defender has struggled immensely down the left with the Argentine.
59 minPiatti causes problems again down the left as he cuts inside his man and feeds Canales in the inside left channel and he takes a touch before firing a left-footed shot at goal, which goes well wide of the post.
61 minSHOT! Cartabia goes for his hat-trick but has his effort blocked by Besle as the ball looked to be heading for the back of the net.
64 minCHANCES! Valencia come so close to adding another goal to their tally as Bernat breaks up field and fires a shot, which looks to be goalbound before Lopar makes the save. But he only parries it as far as Piatti, but he lifts an effort well over the bar.
66 minNater does well to get down the right for St Gallen he has support in the area but he lifts his cross straight into the hands of Guatia, who claims the ball with ease.
69 minOFF THE POST! Parejo is so unlucky as his free kick from 25 yards clatters off the post and bounces to safety with Lopar beaten.
71 minGOAL! Valencia 5-0 St Gallen (Canales)
71 minValencia get their fifth goal of the game from a free kick as Piatti was brought down 30 yards from goal and Canales struck his shot over the wall and into the bottom corner, leaving Lopar with no chance.
74 minGOAL! Valencia 5-1 St Gallen (Nater)
74 minThe visitors get a goal back immediately as Nater converts from close range after collecting Wuthrich's flick on from a cross from the left.
78 minCHANCE! St Gallen go close again as Nushi wins the ball in the midfield and drives at goal and shoots from about 30 yards, his effort looks goalbound but swerves away at the last moment and goes inches wide.
79 minThe visitors have really upped their game in the last five minutes and appear to be playing with a lot more freedom, it's a shame it's far too late for them now. This St Gallen should have showed up in the first half.
81 minValencia are trying desperately to take the pace out of the game to save their goal difference. They were two goals behind St Gallen going into the game, it could prove crucial later in the group stages.
83 minA chipped ball over the top of the St Gallen defence falls nicely for Pabon, he takes a touch but Mantondon comes across to win the ball and clear. The forward was so close to being through on goal, good defending from the St Gallen skipper.
85 minSHOT! Canales looks for his second goal of the game as he fires a free-kick from 25 yards but his effort blasts off the wall and the visitors clear the danger.
88 minCHANCE! Postiga fires wide when presented with a great chance to finish the game on a high for the home side but he lost his composure at the vital moment.
90 minThere will be two minutes of added time.
90 + 2 minFULL-TIME: Valencia 5-1 St Gallen
9.55pmValencia cruise to an easy win over St Gallen thanks to their exploits in their first half when they scored four goals in 20 minutes to put the game to bed. They were by far the better side and it could have gotten much worse for the visitors if the hosts hadn't taken their foot off the pedal after the break.
9.57pmMiroslav Dukic's side now move to second in Group A above St Gallen after their victory tonight at the Mestalla, just a point behind Swansea who are top following their draw with Kuban Krasnador this evening.
10pmThanks for joining our commentary this evening. Make sure to stay tuned for all our coverage of the Premier League and all the other major European teams this weekend, until then, goodbye.