Ighalo opened the scoring straight after halftime with his first goal of the year, running on to a through ball from Troy Deeney and rounding goalkeeper Rob Elliot before slotting home.
Defender Craig Cathcart doubled the advantage when he lashed the ball into the net from close range following a corner.
Jamaal Lascelles pulled one back for Newcastle with a downward header but Steve McClaren's side remained a point adrift of safety.
"We started the game so well and looked in control," McClaren told the BBC.
"The two goals really killed us. It's disappointing -- we fought and came back. We threw everything at them - it wasn’t our day.
"The second goal was from an aimless cross into the box. We then had a mountain to climb. We lick our wounds and go again."
Watford manager Quique Sanchez Flores was pleased with his players.
"We played very well -- it was an energetic team. It was difficult against Newcastle," he said.
"It was a crucial match during the season. Halftime today is the period where we tried to correct things."
(Reporting by Ed Osmond, editing by Ian Chadband)
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