In 1953-54, Aaker was among the many child actors who auditioned for the role of "Jeff Miller" on the original 1954 Lassie series, which later aired as Jeff's Collie. That role went to Tommy Rettig. Two weeks later, Aaker won the role of "Rusty" on The Adventures of Rin-Tin-Tin, co-starring James Brown (1920–1992) as Lieutenant Ripley "Rip" Masters. Aaker and Rettig were friends, and both shows were popular with audiences. The two actors and the two star dogs appeared together in a photograph used on the cover of the July 2, 1955 issue of TV Guide.