Sanchez was out of contract at Arsenal at the end of the season, and joins United after their local rivals Manchester City had pulled out of the running.
Arsenal have done well to land the Armenia global given they could easily have lost Sanchez for nothing in the summer, particularly as the former Borussia Dortmund schemer still had two and a half years left to run on his United contract. Mkhitaryan joins after an up and down 18-month spell with the Red Devils which saw him play an instrumental role in the club's Europa League win but also fall drastically out of favour under Jose Mourinho this term.
"It's a dream come true", Mkhitaryan said, "because I've always dreamed of playing for Arsenal".
Arsenal manager Arsene Wenger said of Sanchez's move: 'January has been hard for us, destabilising.
"I believe we lose a world class player and gain a world class player", Wenger said in his pre-match press conference.
United travel to Huish Park on Friday for a fourth-round tie and Sanchez is eligible to play after sitting out Arsenal's defeat at Nottingham Forest in the competition earlier this month.
The Sanchez-Mkhitaryan deal is the latest move of a January transfer window during which Liverpool sold Philippe Coutinho to Barcelona for 160 million euros and signed central defender Virgil Van Dijk from Southampton for £75 million (85.5 million euros).
The Chile worldwide has scored 80 goals in 166 appearances in all competitions for Arsenal since joining from Barcelona for 35 million pounds in July 2014.
"I would like to say a heartfelt thank you to Manchester United players and staff and especially the fans for your love, passion and support. They can always play together", he said.
Last season, as it became increasingly clear that Arsenal wouldn't be his club for much longer, Sanchez was regularly seen remonstrating with his teammates, who - in his eyes - weren't matching his will to win.
"There have been a lot of young players, he's managed them and they've done very well".
Arsenal boss Arsene Wenger added: "Henrikh is a very complete player".
"Look, I don't want to come back on the history of these negotiations".
"He creates chances, he defends well, he can absorb distances and he's very committed as well", Wenger said.
Manchester United are also responsible for the next three salaries on the Premier League's highest-paid players list, with Paul Pogba's £290,000-a-week contract being signed in 2016 as part of his then world-record £89.5m move from Juventus.
"I must say he's a player who has all the attributes".
Federal Government Reopens After Spending Bill Passes
Senate Minority Leader Chuck Schumer (D-NY) announced his support for the compromise on the chamber's floor Monday afternoon. Treasury futures prices eased and the dollar index against a basket of currencies was slightly weaker.
Economy grows 'solid' 6.6 percent in 4th quarter
Pernia said public construction spending also surged 25.1 percent, offseting the 2.9-percent contraction in private construction. The economy's growth rate in 2010 was at 7.6% but then dropped to 3.7% in 2011.
Bangladesh cruise to easy win vs. Zimbabwe
His googly sparked Bangladesh's collapse from a position of dominance as it appeared set for a big total, thanks to Tamim. Zimbabwe were dealt early blows as Shakib and Bangladesh skipper Mashrafe Mortaza tore through the opposition top order.