Cássio began his career at the academy of Grêmio in his home state of Rio Grande do Sul and was quickly promoted to the club's first team.

[2] Cássio was a Brazilian international from 2007 to 2019 and played at two major tournaments, the 2018 FIFA World Cup and the 2019 Copa América, which Brazil won.

Cássio made his debut in the Brasileirão on 26 October 2006, replacing the injured Rodrigo Galatto in a 2–1 away win over Fluminense.

He spent the remainder of his time at Grêmio as the club's third-choice goalkeeper behind Marcelo Grohe and Galatto, who were also youth graduates.

He went on to make 7 more appearances for Corinthians in the competition, helping the team become champions, including a miraculous save against Vasco da Gama in the second-leg of the quarter-finals.

On 16 December 2012, Cássio helped Corinthians beat Chelsea by the score of 1–0 in the FIFA Club World Cup.

Eventually he did make it to the preliminary squad for the Beijing Games, but lost out a place in the final list to Renan and Diego Alves.