The 20-year-old defender, signed on loan from Tottenham, could hardly have written the script better himself as he scored a brilliant goal nine minutes into his debut against Sheffield United, the club he supported as a boy and where he began his career.
Marc Albrighton made it 2-0 before the break and Villa were able to withstand a second-half fightback by the Blades, for whom Jamie Ward scored from the spot, before Stiliyan Petrov - after Ashley Young's dismissal - secured their place in the hat for the fourth round of the FA Cup.
Gerard Houllier, who had included Robert Pires in his starting line-up despite his fellow Frenchman's critical comments this week, would have been encouraged as Villa made a blistering start at Bramall Lane.
Micky Adams' side looked intimidated and their feeble start was summed up perfectly when Walker strode through to give the visitors the lead.
The right-back slipped away from one challenge just inside the United half and advanced into space before side-stepping Rob Kozluk and slotting home past Steve Simonsen.
It was a tremendous goal but Walker kept his celebrations muted against his old club.
The home side had shown little in response but should have taken advantage when Richard Dunne's weak header allowed in Ward but Brad Friedel spread his big frame to block.
Ched Evans, the Blades' former Manchester City striker, sent a header wide from Leon Britton's free-kick but the tie seemed as good as over when Albrighton volleyed home for Villa in the 33rd minute.
Young made the goal with a trademark burst down the left and lovely dinked cross to the far post that left Albrighton with an open goal to crash in his volley.
Gabriel Agbonlahor then threatened to make it a first-half rout but was denied by a brave save from Simonsen.
The Blades, after a first half that could not have been much worse, started off the second in the best possible way.
In the 48th minute Lee Williamson enticed Carlos Cuellar to trip him inside the box and referee Michael Jones was well placed to award the penalty. Ward smashed it home down the middle - perhaps he had watched Cesc Fabregas earlier on the tv - with Friedel diving to his right.
The goal fired up United and Williamson then drifted an effort onto the roof of the net from a tight angle.
Walker, Villa's hero of the first half, tried a trick on the edge of his own area and was hurried into a pass which ended up at Daniel Bogdanovic's feet and only Friedel's acrobatics prevented further embarrassment for the young debutant.
With 25 minutes left Houllier made a double substitution to try to stop the rot with Petrov and Downing replacing Barry Bannan and Pires.
Villa did look stronger as a result and Agbonlahor used his pace to get ahead of the defence but his low shot hit the side-betting.
Petrov should have sealed Villa's place in the hat for the fourth round draw with two chances in two minutes.
First of all he was put through by Downing and tried to tuck in under Simonsen but the keeper made a good block, then his header from Downing's corner bounced just the wrong side of the post.
United needed some fresh encouragement and received it when Young was given his second booking after a foul on Kyle Bartley.
An overhead kick - by centre-half Johnny Ertl of all people - brought a terrific save out of Friedel.
In injury time Petrov made amends for his earlier miss with a clinical finish following a one-two with the impressive Albrighton.