The Brazilian – who opened his account for the Magpies against Hibernian on Tuesday night – gave Steve Bruce’s side a half-time lead with a prodded finish shortly before the break.
Youngster Longstaff, meanwhile, had been on the pitch for less than a minute when he smashed home a stunning second for United in front of the Gallowgate End.
Ex-Magpie Mathieu Debuchy pulled a goal back for Les Verts late on, but it was too little, too late for the French outfit.
Five minutes in, it was United who engineered the first shooting opportunity of the game. Following a Saint-Étienne corner, Miguel Almirón robbed Yann M’Vila on the edge of the penalty area before driving into the opposition’s half. The Paraguayan twisted and turned as he looked to outwit the visitors’ defence but saw a low-struck effort blocked by William Saliba.
Saliba produced another fine piece of defending midway through the half to deny Joelinton, who took aim after pouncing on a loose ball. Isaac Hayden then gathered Almirón’s through ball but the midfielder’s deflected shot curled past the base of the near post.
After a slight lull, the Magpies nearly created an opening through Jonjo Shelvey’s short corner on 34 minutes, with Fabian Schär’s low strike blocked before it could reach the near corner.
Schär, however, was instrumental in creating the first goal of the game five minutes later. The Swiss curled a fine pass into the channel for Joelinton to chase, with the Brazilian’s light touch taking the ball past onrushing goalkeeper Stéphane Ruffier. The Magpies’ new No. 9 still had a bit of work to do but held off a couple of last-gasp challenges and managed to poke home.
Looking to cement their advantage, Bruce’s side got off to a purposeful start after the break. Almirón tested Ruffier with a strike from just inside the penalty area, before Les Verts’ ‘keeper punched Shelvey’s free kick away from goal. Almirón then seemed set to make the net ripple for the first time as a Newcastle player after bursting through on goal, but failed to keep a driven shot on target.
Just before the hour, Bruce handed debuts to Allan Saint-Maximin and Jetro Willems, who both linked up with Newcastle on Friday. The former was involved immediately, powering down the left flank before checking inside and firing over the top. He brought a low stop out of Ruffier just a few minutes later before creating a chance for fellow substitute Yoshinori Muto, who shot powerfully at the ‘keeper.
However, it was another substitute who wrapped things up for the Magpies with 74 minutes on the clock. Less than 60 seconds after coming on, the younger Longstaff picked up the ball 20 yards out and let fly with a wonderful right-footed strike which hit the roof of the net.
Muto twice could have made the game safe for United during the following exchanges, planting Javier Manquillo’s inch-perfect cross at Ruffier before side-footing wide after bearing down on goal.
Saint-Étienne – fourth in Ligue 1 last term – had offered little in the second half but threatened to make a game of it with five minutes remaining, with Debuchy applying the finishing touch following a corner from Les Verts’ left. However, they were unable to fashion any further chances, as the Magpies rounded off their pre-season campaign on a winning note.
Newcastle United: Martin Dúbravka, Javier Manquillo (Jacob Murphy 83), Fabian Schär (Ciaran Clark 78), Jamaal Lascelles (Paul Dummett 78), Federico Fernández, Rolando Aarons (Jetro Willems 59), Isaac Hayden (Matty Longstaff 74), Sean Longstaff, Jonjo Shelvey (Ki Sung-yueng 73), Miguel Almirón (Allan Saint-Maximin 59), Joelinton (Yoshinori Muto 59).
AS Saint-Étienne: Stepháne Ruffier, Mathieu Debuchy (Sergi Palencia 86), William Saliba (Harold Moukoudi 62), Loïc Perrin (Zaydou Youssouf 62), Wesley Fofana (Aimeen Moueffek 83), Mahdi Camara (Mickael Nade 72), Yann M’Vila (Franck Honorat 83), Denis Bouanga, Romain Hamouma (Loïs Diony 82), Ryad Boudebouz, Robert Berić (Arnaud Nordin 62).
