Cosmo was adopted by dog trainer Mathilda DeCagny from a Jack Russell rescue society in 2007. DeCagny had previously owned and trained other Jack Russell dog actors Moose and his son Enzo, who had played Eddie in Frasier.[1] DeCagny described Cosmo as being skittish and lacking confidence when she first adopted him, but he was very friendly towards people and very playful.[2] When not appearing in films, Cosmo lives with DeCagny and her family in Venice, California.[3]
While on the books of Hollywood pet agency Birds and Animals Unlimited,[4] he made his on screen debut in Hotel for Dogs where he played "Friday".[1] Of the seventy dogs hired for the film, including two doubles for his role, he was described as producer Ivan Reitman's favourite.[5] He subsequently appeared in Paul Blart: Mall Cop.
In Beginners, his fur was dyed using non-toxic vegetable colouring to make him look more similar to Bowser, a dog owned by writer/director Mike Mills as the story was based on Mills' early life.[1] Working with Cosmo inspired actor Ewan McGregor to get a dog of his own. "I was looking for a replacement for Cosmo because I couldn’t stand the idea of not having him around," said McGregor,[1] who adopted a poodle mix called Sid. "I found my dog on the last day of the shoot. He is gorgeous and my little Cosmo replacement."[1] McGregor stated that he takes Sid for walks with DeCagny and Cosmo in Venice on occasion.[1]