McKernan, who played 34 games in six seasons before the Crows delisted him, was snapped up with the Bombers' first rookie selection (No.12 overall).
He trained with St Kilda post-season, but the Saints used their first pick on Port Melbourne midfielder Jack Sinclair, opening the door for Essendon, which was keen to bolster its tall stocks.
"The aim was to get a bloke who could play in the ruck and play forward and was physically ready," Essendon list manager Adrian Dodoro said.
"We felt there was a gap on our list with only the two ruckmen and we just wanted to get some more support there.
"McKernan is a good fit. His best is very good and we understand we've got to get some more consistency in his game, but at 24 we felt he's got a lot of good footy ahead of him."
McKernan explored a return home to Victoria at the end of the 2013 season, but there was little interest and he signed a one-year deal with the Crows.
After playing only the first two games of the season, however, he was sent back to the SANFL where his form fluctuated.
The 24-year-old finished the year strongly to revive his chances of playing on elsewhere, influencing games as a ruckman and catching the Bombers' eye.
He will provide insurance for big men Tom Bellchambers and Jonathan Giles.
McKernan was originally drafted by the Crows with pick No.28 in the 2008 NAB AFL Draft.
