Walcott, who missed most of 2014 with a knee injury sustained in an FA Cup game, volleyed home a shot in the 40th minute to put defending champion Arsenal ahead.
Alexis Sanchez doubled the lead with a fierce 25-yard shot that dipped in via the underside of the crossbar before Per Mertesacker scored with a header in the 62nd. Olivier Giroud completed the win in injury time.
After a nine-year trophy drought, Arsenal has now won the Cup two years in a row, overtaking Manchester United’s record of 11 titles.
The win also meant Arsene Wenger tied the record for Cup victories by a manager with six, level with Villa’s George Ramsay, who won his last title in 1920.
This victory was a lot more comfortable than last year’s, when Arsenal had to come from two goals down to beat Hull 3-2 in extra time. Saturday’s result never looked in doubt as Arsenal dictated the game from the start and Villa — which had to fight against Premier League relegation until the penultimate round of the season — didn’t have a clear scoring chance in the entire game.
Arsenal, meanwhile, had created a handful before Walcott finally scored.
Shay Given made a good one-handed save to deny Laurent Koscielny’s header in the 15th minute, Aaron Ramsey lifted a volley over the bar in the 20th and Walcott was denied from point-blank range by a diving clearance from Kieran Richardson in the 24th.
Walcott has struggled for playing time even since returning to full fitness, with just four Premier League starts this season. However, after scoring a hat trick in the last game of the league season, he was preferred up front to Giroud. His speed proved decisive after Nacho Monreal sent in a cross from the left that Sanchez met by the far post and headed back across the area. Walcott beat two defenders to the ball and volleyed home inside the near post, before racing with both arms outstretched to celebrate with the Arsenal supporters.
The Gunners kept pouring forward after the break, with Mesut Ozil dictating play in midfield. Sanchez’s goal was a pure solo effort, though, as he picked the ball up 35 yards from goal, drifted forward and unleashed a strike that dipped past Given in the net.
Prince William, an Aston Villa fan, shook hands with the players from both teams before the match, but even he had a disappointed look on his face by the time Mertesacker headed in the third.
After Walcott was taken off for Giroud to a standing ovation, the Frenchman was left unmarked to tap home a cross from fellow substitute Alex Oxlaide-Chamberlain in injury time.