A February 2015 draft ranking by Mel Kiper, Jr. rated him as the eighth best quarterback in the 2015 NFL draft. NFLDraftScout.com rated him the 30th best quarterback in the draft. Some thought that Cato's size would hurt his draft chances.
On May 16, 2015, Cato signed a two-year deal with the CFL's Montreal Alouettes. He started his first game for the Alouettes on July 3 against the Calgary Stampeders after Jonathan Crompton and Dan LeFevour sustained injuries in Week 1 against the Ottawa Redblacks. He threw a touchdown pass on his first drive as an Alouette. In the first half, Cato went 12–15 for 146 yards and 2 touchdowns. He continued in the second half with another touchdown scoring opening drive. Cato finished the game 20–25, 241 yards and 3 touchdowns for his first CFL win. For his efforts, Cato was named player of the game, with TSN analyst and Canadian Football Hall of Fame quarterback Matt Dunigan characterizing his performance as "phenomenal." He was named Player of the Week by the league for his performance against Calgary. Cato was one of three players chosen as Player of the Month for July in the CFL. Cato played in 12 games in 2015 for the Alouettes, completing 174 out of 215 pass attempts (80%) with 9 touchdowns and 9 interceptions.