Lah was born in Illinois. He worked briefly at Walt Disney Studios before joining MGM in the early 1940s. His first work at MGM was for Hugh Harman and Rudolph Ising. After Harman and Ising left, Lah worked briefly on Bill Hanna and Joseph Barbera's Tom and Jerry series. He then joined Tex Avery's unit as lead animator, where he remained until the studio closed in 1957. During Avery's absence in 1950-51, and after Avery left MGM in 1953, Lah was given co-directorial credit on many animated shorts.
After he left MGM, he briefly rejoined Hanna-Barbera at their TV cartoon studio as an animator, then joined Quartet Films, a commercial animation studio that created television commercials for Kelloggs and Green Giant Foods.
He was an active member of ASIFA-Hollywood, serving on the board for several years. In 1984, Lah received the Winsor McCay Award for his lifetime of work in the animation field.