The 27-year-old forward fuelled speculation over his future this week by stating he will wait and see how Chelsea's close-season transfer policy develops before committing to a new contract at Stamford Bridge.
But he was all business at England's national stadium as he turned the first half into an ordeal for Phil Jones – the United defender whose clumsy foul meant Hazard won and converted a 22nd-minute penalty.
United improved upon a tepid showing after the break and Alexis Sanchez had a goal ruled out for offside, but even the late introduction of Romelu Lukaku on his return from an ankle injury could not force an equaliser.
It meant a sweet victory for Antonio Conte over his rival Jose Mourinho on what could prove his farewell outing as Chelsea coach, with the FA Cup a notable consolation after their Premier League title defence slumped to a fifth-place finish.
Jones gave away possession and Tiemoue Bakayoko set Hazard running at the backpedalling England defender, who he skipped past and forced De Gea to save with his right boot at the near post.
Bakayoko and Sanchez had half-hearted penalty appeals turned down but there was little doubt for referee Michael Oliver when he later pointed to the United spot.
An utterly hapless start to the game continued for Jones, who blundered through Hazard as the Belgium star darted around him after bringing Cesc Fabregas' lofted pass under his spell.
Conte was furious on the touchline after Jones was only yellow carded – in line with the International Football Association Board update brought in from last season – but Hazard was a picture of calmness as he stroked home, sending De Gea the wrong way.
Jones had a chance to make amends but headed wide at the far post from Ashley Young's left-wing cross, with United belatedly arriving as an attacking force in the closing minutes of the half.
Antonio Rudiger blocked well from Marcus Rashford after Paul Pogba drove a counter-attack in stoppage time.
Mourinho's side enjoyed their first sustained spell of pressure early in the second half, with Ander Herrera liberated from attempting to shackle Hazard and lending greater impetus to United's midfield play.
Thibaut Courtois beat away a drive from Rashford and Pogba saw a deflected 57th-minute shot loop behind, before Nemanja Matic scuffed wide wastefully against his former club.
The pressure looked to have told in the 63rd minute but Sanchez was fractionally offside – a decision confirmed by VAR – when he prodded in after Courtois denied Jones.
Chelsea attacks had become a rarity by the time N'Golo Kante marauded forward and played in Marcos Alonso. The wing-back was denied by his countryman De Gea and Young escaped penalty claims for handball as Chelsea followed up.
Courtois denied Rashford one against one before the England forward made way for Lukaku, and Mourinho would no doubt have preferred a glorious 82nd-minute headed chance fell to the Belgian as opposed to Pogba.
United's record signing was unmarked but directed substitute Anthony Martial's corner wide, condemning his boss to a trophyless campaign.
Key Opta stats:
- Chelsea have won their eighth FA Cup and their first since 2012, level with Tottenham - only Manchester United (12) and Arsenal (13) have won more.
- Jose Mourinho has lost his first cup final in charge of an English club, after winning each of the previous six (4 x EFL Cup, 1 x FA Cup, 1x Europa League).
- United mustered 18 shots in the game; they last attempted more shots without scoring in a match in all competitions in October 2016 (38 v Burnley in the Premier League).
- Eden Hazard's penalty was the first scored in an FA Cup final since Ruud van Nistelrooy for United versus Millwall in 2004 (excluding shoot-outs).
- Thibaut Courtois attempted 42 passes in the game, more than any of his outfield team-mates.