Alan Shearer missed a penalty as Magpies boss Bobby Robson endured a nightmare return to his old ground, with United's miserable winless streak in the capital now stretching to 27 matches.
Newcastle's Republic of Ireland World Cup hero Shay Given was beaten twice in 10 first-half minutes as Fulham scored two candidates for goal of the month on the way to their third straight victory.
First, Saha - who endured a miserable six-month loan at Newcastle in 1999, scoring just one goal - played a one-two with Sylvain Legwinski and smacked home a sensational 25-yard curling shot.
Then Legwinski switched from provider to scorer when he thumped home a fine second goal after a precise Steed Malbranque through-ball.
Gary Speed gave United hope with a thumping header, but a predatory strike from Barry Hayles wrapped up Fulham's fully-deserved 3-1 triumph.
Speed and Shearer were both off-target early on, heading wide and over, before a comedy of errors in the home defence almost handed the visitors the lead after 21 minutes.
Former Newcastle defender Alain Goma fluffed a headed clearance and Craig Bellamy advanced on goal.
As he skipped past Andy Melville, Fulham goalkeeper Edwin van der Sar lost his footing before recovering to smother Bellamy's shot.
Play then switched immediately to the other end and Saha scored a fantastic goal against his former club.
The 23-year-old nodded the ball wide to Legwinski on the right, and he played the ball back to his fellow Frenchman, who took two touches and unleashed a brilliant 25-yard left-foot shot which soared into the top left corner of Given's net.
And 10 minutes later, the Cottagers doubled their lead with another excellent goal.
This was a team effort, with John Collins and Saha involved in the build-up before Malbranque slipped a delightful ball inside United left-back Rob Elliott and into Legwinski's path.
The midfielder powered the ball past Given's outstretched right hand and into the back of the net for number two.
Fulham were now running the Newcastle backline ragged, with only two blocks from Given preventing first Malbranque and then Hayles making it 3-0.
Saha should have put the match beyond Newcastle five minutes after the break. Malbranque pounced on a mis-hit defensive pass towards Andy O'Brien and put Saha clean through.
Given made a vital block as the ball luckily hit his elbow and deflected to safety.
Newcastle halved the deficit after 66 minutes with the third fine goal of a cracking match.
Aaron Hughes swung over a pacey cross from the right wing and Speed climbed above the home defence to crash a fine header into the bottom left of van der Sar's net.
However, the Cottagers restored their two-goal cushion in the 71st minute when a whipped Malbranque corner from the left was flicked on.
Hayles beat Given to the ball inside his six-yard box and gleefully headed Fulham's third.
Shearer then had a wonderful opportunity to make the score 3-2, but the former England forward's penalty was brilliantly saved by van der Sar.
Goma was harshly adjudged to have brought down Laurent Robert inside the box - and Dutch international van der Sar dived to his right to tip Shearer's effort onto the post.The United skipper could not convert the rebound and Newcastle's capital punishment continued.