Leach had been coach of Providence, the American Hockey League affiliate of the Boston Bruins, since 2017-18. He was an assistant there in 2016-17 and previously held the same position with Wilkes-Barre/Scranton, the Pittsburgh Penguins' AHL team, in 2015-16.
McFarland had been coach and general manager of Kingston in the Ontario Hockey League since May 8, 2020.
"We're very excited to add two talented hockey minds in Paul and Jay to our inaugural staff," coach Dave Hakstol told the Kraken website. "Paul's work ethic and ability to communicate with players to give them the tools to be at their best, along with Jay's leadership and ability to coach and develop NHL talent, will be great additions to our team."
Hakstol was hired as the Kraken's first coach June 24. He said McFarland will focus on forwards and the power play. Leach, a former NHL defenseman, will be responsible for the defensemen after helping develop Bruins defensemen Charlie McAvoy, Jeremy Lauzon and Connor Clifton.
Leach played 70 NHL games for the Bruins, Tampa Bay Lightning, New Jersey Devils, Montreal Canadiens and San Jose Sharks after he was selected by the Phoenix Coyotes in the fifth round (No. 115) of the 1998 NHL Draft.
"I enjoyed all of it [in Providence] and learned a lot," Leach said. "It is exciting to be part of a brand-new NHL team from the ground up, especially under Dave and [general manager] Ron [Francis] in one of the best cities I have ever visited.
"There is something to be said about coaching the best players in the world at the highest level."
McFarland and Hakstol were assistants with the Toronto Maple Leafs under Mike Babcock and Sheldon Keefe during the 2019-20 season. McFarland also was an assistant with the Florida Panthers from 2017-19.
"It's a once-in-a-lifetime opportunity to work with this organization, starting with Ron, as part of the expansion process," McFarland said. "Anytime you work with somebody with the schedule demands of the NHL, you get to know that person. Dave and I got along great.
"I look forward to working with Dave again, plus Jay and the rest of the staff. As coaches, we're most excited about getting on the ice for training camp with our players. We will be prepared. We will help Dave realize his vision."
The Kraken will enter the NHL as its 32nd team next season. The 2021 NHL Expansion Draft will be held July 21, and Seattle has the No. 2 pick in the first round of the 2021 NHL Draft on July 23.
"Paul and Jay are both bright, young coaches who are well respected for their abilities," Francis said. "They will work well with Dave in building a winning culture for our franchise."
