It was an afternoon of 'what might have been' after the Gunners had entered 2016 top of the table and looking in good shape to claim their first title in 12 years.
Instead the 4-0 victory over the long-since relegated Villa saw Arsenal leapfrog Spurs to finish second to champions Leicester as Giroud picked the perfect time to show Arsene Wenger he can lead the line at the Emirates Stadium going into next season, with a last-gasp Mark Bunn own goal sealing the win.
The win, coupled with Tottenham's capitulation at Newcastle, means Arsenal have not finished below their neighbours since 1995 and were able to toast 'St Totteringham's Day', the name fans have given to the point where they are assured of finishing above Spurs, once again.
It could have been a simple end-of-season knockabout but, with second spot still up for grabs, Wenger's side at least had something to play for and they needed just five minutes to breach Villa's leaky defence.
Nacho Monreal broke down the left and crossed for an unmarked Giroud to head home past a despairing Bunn - although Mesut Ozil had appeared to run the ball out of play in the build-up.
Villa came close to levelling soon after but Andre Ayew's ball across the Arsenal goal was cleared by Gabriel after the forward had burst past Hector Bellerin and into the box.
Alexis Sanchez arrowed a shot wide as Arsenal looked to double their advantage before the break, although arguably the loudest cheers of the half came when news filtered through of Newcastle taking the lead over Spurs.
With England boss Roy Hodgson watching in the stands, Jack Wilshere should have scored when picked out six yards out but the 24-year-old - starting his first game in almost a year - could only turn his chance into the side-netting.
The Gunners were almost made to pay as Scott Sinclair was allowed to get a shot away, but it was wide of Petr Cech's far-post and Jordan Lyden's cutback could not find a Villa shirt.
The second half was a bland affair as the hosts struggled to create chances, while Villa did not look threatening on the rare occasions they committed men forward.
Wilshere's afternoon came to an end when he was replaced by Mohamed Elneny after 68 minutes and the Egypt midfielder was guilty of spurning a great chance as he wildly lashed a volley over from close range.
The killer goals Arsenal so desperately coveted came within a two-minute window in the closing stages; firstly the Frenchman swept home Ozil's low cross before staying onside to beat Bunn to Bellerin's tantalising throughball.
A fourth goal in stoppage time, an own goal by Bunn rather than a most-fitting finale for departing club captain Mikel Arteta - whose effort was tipped onto the bar before hitting the unwitting Villa stopper and rolling over the line, wrapped up the afternoon.
TWEET OF THE MATCH
"I apologise that it's late this year but Happy St Totteringhams day everyone#StTotteringhamsDay" - Excited TV presenter Alex Brooker (@alex_brooker) revels in Arsenal finishing above Tottenham
PLAYER RATINGS
ARSENAL
Petr Cech: 6
Gabriel: 7
Laurent Koscielny: 7
Jack Wilshere: 7
Mesut Ozil: 7
Olivier Giroud: 8
Alexis Sanchez: 8
Nacho Monreal: 7
Santi Cazorla: 7
Hector Bellerin: 7
Francis Coquelin: 7
SUBSTITUTES
Mikel Arteta (on for Ozil, 89): 7
Joel Campbell (on for Cazorla, 89): 6
Mohamed Elneny (on for Wilshire, 68): 6
ASTON VILLA
Mark Bunn: 6
Leandro Bacuna: 5
Idrissa Gana: 5
Scott Sinclair: 5
Ashley Westwood: 5
Joleon Lescott: 5
Jordan Ayew: 5
Carlos Sanchez: 5
Jordan Lyden: 5
Aly Cissokho: 5
Kevin Toner: 5
SUBSTITUTES
Micah Richards (on for Lescott, 64): 5
Jack Grealish (on for Lyden, 74) : 4
STAR PLAYER
Olivier Giroud: A hat-trick for the French striker was all Arsenal needed to steal second place from their rivals Tottenham
After scoring his first goal after five minutes, Giroud had a quiet afternoon until, spurred on by a loud crowd, he bagged his second and third just two minutes apart.
MOMENT OF THE MATCH
In a relatively quiet game, the moment of the match came from the fans
After 39 minutes news rippled around the ground that Newcastle had gone 2-0 up against rivals Tottenham meaning if results stayed as they were they would finish the season in second, ahead of Spurs
The noise from the crowd as Newcastle's third goal went in on 73 minutes was electric and just moments later Arsenal scored their second of the afternoon
The change of atmosphere seemed to wake the home side up and their game immediately improved.
VIEW FROM THE BENCH
Striker Theo Walcott was left out of the squad by Arsene Wenger due to a "slight hamstring injury"
Many had expected him to be involved with Danny Welbeck being ruled out for nine months
Giroud was chosen to play upfront and after his initial goal had a very quiet afternoon two goals in two minutes secured the Gunners second in the league.
MOAN OF THE MATCH
Giroud's fifth-minute goal killed the majority of the game
The striker headed home Monreal's cross and after that the Gunners sat back, passing the ball around the back
Arsenal had a couple of shots on target but looked more than happy to tire Villa out
It was not until the fans let their team know Tottenham were losing did they wake back up
Source : PA
Source: PA