The Armenian arrives at the club for free with Alexis Sanchez going in the opposite direction as the latter finally departs the Gunners after a drawn out transfer saga.
Dream come true
Mkhitaryan joins Arsenal having had a largely unsuccessful spell with United, making 63 appearances and scoring 13 times after moving from Borussia Dortmund in 2016.
Upon signing for the club, the 29-year-old revealed his delight. "I’m very happy that we could finish this deal and I’m very happy to be here.
"It’s a dream come true because I’ve always dreamed of playing for Arsenal. Now that I’m here, I’ll do my best for this club to create history.”
He meanwhile spoke of his admiration for the club - something which has been well documented in the past. "I always loved the way that Arsenal played and the way that Arsène Wenger has managed the team.
"Of course I was in love with the game of Thierry Henry. I loved the way he played - and maybe that was the reason to support Arsenal," Mkhitaryan commented.
Sanchez saga draws to a close
For Arsenal they will be relieved to finally see a deal for Sanchez reached - who was due to run out of contract at the end of the season. After turning down a big bid from Manchester City last summer the Citizens surprisingly opted not to come back in for him in January.
That left the door open for city rivals Man United to swoop in, who rather than paying a reported £35m asking fee for the Chilean, agreed to swap Mkhitaryan instead.