The Magpies conceded twice in quick succession at Ashton Gate as Sam Greenwood's curling free-kick fired the Championship outfit ahead before Lorent Tolaj doubled the Robins' advantage before the half-hour mark, despite the Swiss striker being adjudged to foul Sven Botman in the build-up.
The visitors, forced to withdraw Fabian Schär in the early stages due to injury, fell further behind shortly after the restart following Max Bird's strike and despite Harvey Barnes reducing the deficit, Botman's own goal restored Bristol City's three-goal cushion in the closing stages.
Eddie Howe made three changes from Saturday's 1-1 draw against Gateshead as goalkeeper Ewen Jaouen was handed his first start for the Magpies following last month's transfer from Ligue 2 side Stade de Reims while Botman and Anthony Elanga replaced Alex Murphy and Leo Shahar in the starting 11.
Jaouen produced the opening save of the night as the 20-year-old shot-stopper denied Greenwood in the opening stages. The Sunderland-born forward beat the offside trap following Bird's through pass but after the resulting effort was blocked, the French 'keeper quickly smothered the loose ball.
The entirety of Ashton Gate rose to conduct a minute's applause in honour of the late Kevin Keegan in the seventh minute, a nod to the number he wore during a terrific playing career, with the former Magpies player and manager's image shown on the stadium screens.
Newcastle, backed by 3,400 away fans, were unfortunate not to take the lead shortly after as Sean Steur's teasing corner was headed towards goal inside a crowded box, with City goalkeeper Sam Tickle tipping the ball over his own crossbar.
The visitors were forced into an early substitution as Schär, making only his second start since suffering an ankle injury in January, was withdrawn after an awkward fall.
Sean Neave, on target in Newcastle's 3-0 friendly win against Darlington, went close to netting his second goal of the pre-season campaign.
The Winlaton-born forward latched onto Jacob Murphy's drilled cross inside the box but his smart first-time effort was pushed around the post by Tickle.
Murphy took his turn in front of goal after Cameron Pring's headed clearance fell into the Magpies skipper's path but his fierce volley flew wide of the left-hand post.
Newcastle trailed in the 26th minute through Greenwood's clinical free-kick. After the Robins forward was tripped on the edge of the box by Jacob Ramsey, his resulting set piece flew into the net via the underside of the crossbar.
United were dealt a further blow in the 29th minute as Tolaj extended the hosts' lead in controversial fashion. Despite the Swiss frontman pushing Botman to the floor, Tolaj was allowed to race towards goal by referee Paul Tierney before slotting the ball beyond Jaouen to the frustration of the visitors.
With the half-time whistle nearing, Newcastle were appealing for a penalty as Elanga advanced beyond Lisav Eissat before going down inside the box after a push from the Romanian defender but the decision was waved away by Tierney.
United almost halved the deficit in the opening minute of the restart as Neave struck the crossbar. After Barnes' cross was knocked down into Neave's path, the England Under-19 international was denied a first goal at senior level as his effort clipped the woodwork.
In the 50th minute, Newcastle conceded a third as Bird scored for the hosts. After collecting Jason Knight's pass, the Robins midfielder dangerously advanced into the box before slotting the ball into the far right-hand corner beyond goalkeeper Nick Pope, introduced at the restart.
Botman was unable reduce the arrears after the hour mark as the Dutch defender headed over the bar from Lewis Hall's in-swinging free-kick, with Barnes eagerly poised at the back post to fire an attempt towards goal.
Barnes, however, managed to get his name on the scoresheet in the 78th minute, placing a low effort into the net after a move involving Ramsey and 17-year-old forward Matheos Ferreira, the latter a product of the Magpies' Academy who made his first-team debut in last weekend's 1-1 draw against Gateshead.
Newcastle, though, suffered another setback in the 81st minute as Botman netted an own goal, turning Sam Bell's cross from the left flank into his own net.
The Magpies head to south-east Spain later this week to step up their preparations for the new season, which includes a visit to the iconic Mestalla Stadium to face La Liga side Valencia on Saturday 8th August (kick-off 8pm BST) in their annual Trofeu Taronja (Orange Cup) game.
Newcastle United: Ewen Jaouen (Nick Pope 46), Lewis Hall (Dylan Charlton 90), Sven Botman, Fabian Schär (Alex Murphy 20), Harvey Barnes, Sean Steur, Anthony Elanga (Matheos Ferreira 74), Jacob Murphy (c) (Leo Shahar 82), Joe Willock (Mason Miley 90), Jacob Ramsey, Sean Neave (Park Seung-soo 56).
Subs not used: Aidan Harris, Anthony Munda, Miodrag Pivaš, Trevan Sanusi, Rory Finneran.
