Born in Seattle and raised in the Bronx by his widowed mother Mabel, Pratt graduated from the Pratt Institute in Brooklyn. He became an artist at Walt Disney Studios in 1933 before joining Warner Bros. Cartoons, along with fellow animators Frank Tashlin, Bill Melendez, Emery Hawkins, Basil Davidovich, Maurice Noble, Cornett Wood, Ted Bonnicksen, and Jack Bradbury after the Disney animators' strike in 1941. At Warners, he served as a layout artist providing background layouts and character poses from 1945 to the early 1960s. Working closely with director Friz Freleng, Pratt's Warner Bros. resume includes the Oscar-winning cartoons "Tweetie Pie", which introduced the duo of Sylvester and Tweety, "Speedy Gonzales", where Freleng and Pratt redesigned the character into his modern incarnation, and "Birds Anonymous". Pratt directed "Señorella and the Glass Huarache", a Looney Tune released in 1964 after the studio closed its animation division.