On August 23, 2018 he was named one of the party's two Deputy Leaders alongside Sara Singh, and critic for Agriculture and Food, and for Rural Development.

[1] As of August 11, 2024, he serves as the House leader of the Official Opposition and as critic for Agriculture and Food, for Rural Affairs.

[3] Prior to entering politics he was president of the Temiskaming Federation of Agriculture, a farmer's lobby group.

Vanthof was re-elected for a third time in the 2022 provincial election, defeating Progressive Conservative candidate Bill Foy.

In opposition, he served as his party's critic for Natural Resources and Agriculture, Food and Rural Affairs.