The three-times Grand Slam champion needed time to settle into his game during a tightly-contested opening set but managed to raise his level in the tiebreaker and never looked back as he beat Sonego for the second time this year.
"It wasn't always the prettiest tennis and I was just trying to put the ball in difficult positions for him and hope he doesn't come up with great shots," three-times champion Murray said during his on-court interview.
"He's got a huge game, hits his forehand really big, but in the wind like this it's not that easy to play really high-power, high-pace tennis so I just tried to put balls in awkward places and hope for the best."
The former world number did well to save a pair of set points in a tight opening set which he finally wrapped up in 90 minutes with a dominant display during the tiebreaker.
The Scot then raced through the second set, which he closed out with a third consecutive break on his first match point when Sonego sent a forehand into the net to end a match that lasted two hours and nine minutes.
Up next for Murray will be either 10th-seed Canadian Felix Auger-Aliassime or Australia's Max Purcell.
For Murray, who was coming off a round-of-16 loss to Taylor Fritz last week in Washington, the win over Sonego marked his first in Toronto since lifting the title in 2015.
