Which voice actor voiced both Barney Rubble and Dino?

Mel Blanc voiced both Barney Rubble and Dino in The Flintstones.

Blanc was one of Hanna-Barbera’s key voice performers on the original series. He supplied Barney’s distinctive voice and also performed the vocal sounds of Dino, Fred and Wilma’s enthusiastic pet dinosaur. Alan Reed voiced Fred, making Blanc and Reed the central male voice pairing of the show.

Blanc’s work on the series continued through most of its 1960–1966 run. After a near-fatal car accident at the end of the first season, he briefly recorded from a hospital room, and Daws Butler filled in as Barney for several season-two episodes. That temporary substitution does not change Blanc’s status as the principal performer associated with Barney.

The question refers to the original television series, not every later Flintstones production. Other performers—including Henry Corden and Frank Welker—later took on related roles, but neither replaces Mel Blanc as the canonical answer for voicing both Barney and Dino in the original show.