Jose Mourinho's side suffered a second successive defeat on the road and only their third in 29 league meetings with the Magpies to stay 16 points behind league leaders Manchester City.
Mourinho had failed to win in six previous visits to St James' Park in England's top flight and his players rarely looked likely to bring that run to an end in another hugely disjointed display.
Chances were scarce throughout the contest but Anthony Martial and Alexis Sanchez wasted clear openings, while Romelu Lukaku had a goal disallowed, before Ritchie capitalised on some slack defending in the 65th minute.
The visitors offered some late pressure but could not find a way past debutant goalkeeper Martin Dubravka, meaning their chances of catching City look almost gone with 11 matches left to play.
Newcastle, meanwhile, climb to 13th in the table, two points clear of the relegation zone.
Jonjo Shelvey brought a fine one-handed save out of David de Gea just five minutes in, as Newcastle made a bold start against a team they last beat at home in the top flight in January 2012.
The visitors began to get a grip on possession but their attacking play lacked any real inspiration in the first half-hour, with Jesse Lingard producing their first shot on target 32 minutes in.
Martial wasted a glorious chance before the break, though, racing through one-on-one against Dubravka after a brilliant Nemanja Matic pass, only to shoot straight at the Newcastle goalkeeper.
It was a rare moment of quality from United and they were fortunate not to concede a penalty before the interval, with referee Craig Pawson unmoved after Dwight Gayle when to ground after being caught by Chris Smalling.
The visitors did have the ball in the net 52 minutes in but Lukaku was penalised for a push on Florian Lejeune as he headed into the net, although Mourinho's men were at last beginning to look a consistent threat.
Sanchez came close with a good run and shot but wasted the best chance of the match 56 minutes in, rounding Dubravka after collecting Lukaku's throughball only to see Lejeune block his goalbound effort after he paused in front of an empty net.
It proved a costly miss. After Chris Smalling conceded a free-kick for diving near the halfway line, Shelvey sent a simple long ball into the box, Gayle cushioned a loose ball into Ritchie path and he swept home completely unmarked from 12 yards out.
3 - Manchester United have had three players booked for dives in the Premier League this season (Valencia, Herrera, Smalling), a joint high along with Bournemouth. Splash.— OptaJoe (@OptaJoe) February 11, 2018
Pogba, who looked to have struggled with an injury from the outset, was taken off along with Lingard as Mourinho threw on Michael Carrick and Juan Mata, and the latter's corner led to Martial seeing two efforts from 12 yards blocked on the line by the covering defence.
That was as good as it got for the visitors, though, who must now look for positive results against Huddersfield Town in the FA Cup and Sevilla in the Champions League to keep their trophy hopes for the season truly alive.
Key Opta Stats:
- Newcastle enjoyed their first win in nine Premier League home games (D3 L5), since beating Crystal Palace in October.
- Manchester United have lost five games in the Premier League this season, equalling their tally for the whole of last season.
- Matt Ritchie ended a run of 43 shots this season without scoring in the Premier League, which was the longest such run in the division.
- Dwight Gayle has had a hand in a goal in both of Newcastle's Premier League clashes with Manchester United this season, scoring at Old Trafford in November before assisting Matt Ritchie at St James' Park.
- The Red Devils have won only one of the nine Premier League matches this season after conceding the opening goal of the game (D3 L5), courtesy of a 4-1 win at home against Newcastle in November.