The 19-year-old Little Benton Academy product started in central midfield, wearing the number four shirt, at Wycombe Wanderers' Adams Park and he impressed during a 72-minute outing before being replaced by Flynn Downes of Ipswich Town.
Reading's Danny Loader opened the scoring after a goalless second half before a brace from Manchester City's Ian Poveda-Ocampo sealed a second win in six days for England at this level.
Longstaff had won his first cap as a second-half substitute in Thursday's 4-0 victory in Portugal, and, speaking to NUFC TV after the game, admitted: "You always want to play for your national team - it's a big thing - and to get to do it tonight and get my first start was pretty big."In Portugal I came off the bench and we were 4-0 up at the time, so coming on there was pretty comfortable. Tonight, getting your first start, it's a bit quicker. It was a bit tougher, the way (Iceland) set up, but it was good to finally unlock them."
Meanwhile, Martin Dúbravka kept a clean sheet for Slovakia as they maintained their hopes of qualifying for EURO 2020.
The Newcastle goalkeeper played the full game in a 2-0 win over Azerbaijan in Trnava, and while Wales' victory over Hungary saw Ryan Giggs' side take Group E's final automatic qualifying spot, Dúbravka's side will now go into the play-offs.
In the CONCACAF Nations League, USA won 4-0 away to Cuba with DeAndre Yedlin in the starting line-up. The United right-back played for 59 minutes as his side overtook Canada at the top of Group A to qualify for next summer's knock-out stages. They will take on Honduras at the semi-final stage in June.
Miguel Almirón played the full game for Paraguay as they took on Saudi Arabia in Riyadh, but there was no goal on this occasion for the Magpies man, who had netted in two of his last three international outings, as it ended 0-0.