Perreault had six goals and 17 points in 39 games for the Capitals in 2012-13. His best season was the year before, when he collected 16 goals and 30 points in 64 games.
The move will enable the Capitals to have more flexibility below the salary cap if they choose to keep 2012 first-round pick Tom Wilson instead of sending him back to the Ontario Hockey League. The addition of Mikhail Grabovski and potentially Wilson could have left Perreault as a healthy scratch in Washington.
The 25-year-old Perreault will count $1.05 million against the cap in the second of his two-year deal. Perreault will be reunited with former Capitals coach Bruce Boudreau in Anaheim.
Mitchell, 28, has never played in the NHL. He had 10 goals and 14 points for Norfolk in the American Hockey League last season.