“The plan was always to get it done early in the year so I could focus on the footy,” McDonald told FoxSports.
“North came to me not too long ago and we did it pretty fast. I’m very excited to sign for the next two years.”
McDonald has played 39 senior games since making his debut in 2014.
Garner, who also signed a two-year contract extension, has played 15 games since being selected with pick No.15 in 2012 National Draft.