Thornton will start Monday's game against the Nationals, Rob Longley of the Toronto Sun reports.
The 26-year-old was able to secure a rotation spot during summer camp and will take the mound for the fourth game of the season. Thornton made his major-league debut in 2019 and had a 4.84 ERA, 1.41 WHIP and 149:61 K:BB over 154.1 innings (32 appearances).