The 28-year-old has had a distinguished career with Exiles since joining 10 years ago, playing 171 matches and scoring over 50 tries.
But by his own admission, the last year has been far from his best.
Banned on four separate occasions in 2011, the full-back was then suspended from the England Saxons elite player squad earlier this year after he was arrested following an alleged assault outside a nightclub.
And with Tom Homer proving a more than able replacement in Armitage's absence, the 28-year decided the time was right to call time on his Irish career and link up with brothers Guy and Steffon in Toulon.
"With everything that has gone on, it has not been a good year for me," Armitage explained to the Chronicle.
"Missing out on the England squad and then everything else that happened too, I'm my biggest critic and I admit that I haven't been anywhere near my best this season.
"And then I look around and there are a lot of other talented players in the squad. You look at Tom Homer, Saliosi Tagicakibau and Topsy Ojo and I can't see a spot for me in the side.
"When I started at London Irish I always said that it was my mission to finish my career at the club, and I haven't done that. But it's a good time for me to leave, the time is right."
And Armitage has no doubts that Homer, 22, will be the perfect heir to his full-back position in years to come at the club.
"Tom Homer is definitely the next full-back here. He's got a great kicking game, he's brave under the high ball, and credit to him for everything he has done," he said.
"Hopefully he can continue doing what he is doing and not only be the top point scorer in the Premiership but one of the top players."
Alongside Armitage this Saturday, a number of other players will also bid farewell to the Madejski, including Bob Casey, Nick Kennedy, Paul Hodgson and Dan Bowden.
And England international Armitage insists Irish will do everything possible to end the season with a victory against Gloucester.
"It's going to be an emotional day," he said. "Things haven't gone quite according to plan this year but we had a good win last time out against Worcester and we want to finish the season on a high, not only for the guys that are leaving but for the guys who are staying for next season. This is a really good team going forward.
"As a group we need to win to prepare for next season properly. Gloucester are always tough opponents, no matter their recent results or form, they will be coming to Reading looking to spoil our party.
"It will be a physical battle, it always is against them, we know what to expect and we have to be ready."
Gloucester go into the game on the back of a five-match losing streak and without a permanent coach after Bryan Redpath resigned earlier this month.
But they beat Irish by three points when the sides met at Kingsholm earlier in the season and will be keen to end their campaign on a positive note.
For Armitage, the match will mark an emotional end of his time at Irish but also signal the start of a new challenge with Toulon, one he is excited about.
"London Irish have been brilliant with me, with the support they have given me, I'm going to miss it but I am looking forward to Toulon now," he said.
Asked whether he hopes to play for England again, Armitage said he would not rule anything out, but that it is not at the forefront of his mind at the moment.
"I never say never to anything and never give up, I'd love to be involved with England again," he commented.
"But for now I need to go and focus on my game at Toulon and then whatever will happen will happen."
* Follow live coverage of London Irish's clash with Gloucester on the Reading Chronicle website from 1.30pm on Saturday.