At the end of the first film, when Woody and Sid's mutant toys decide to rescue Buzz by scaring Sid, he becomes very frightened of toys, and Woody pulls the last straw on him by coming alive and telling him to take good care of his toys or else ("From now on, you must take good care of your toys! Because if you don't we'll find out, Sid. We toys can see everything. So play nice!"), causing Sid to panic and run back into his house and into his room. He seems to be the only human depicted in the film to have witnessed toys come alive. It is also still not implied if he & Andy ever were friends or enemies or if they possibly didn't know each other.
Sid does not appear in the second film, but is mentioned once by Buzz during the toys' mission to rescue Woody from Al McWhiggin.
A grown Sid makes a cameo appearance in the third film as a garbageman with a small beard ; he is recognizable by his Zero Skull T-shirt. It is not revealed if he still thinks that the toys come to life or if he has suppressed that memory. It is also unknown whether he still likes to destroy toys or if he has redeemed himself from that. He is only heard scatting and beating to heavy metal music, leaving him with no spoken dialogue, while taking trash to his truck. Notably, when Woody and the other toys (exceptions: Mrs. Potato Head, Aliens, Jessie, Bullseye, Prospector, and Wheezy) saw him again at the garbage dump, they didn't even react to seeing him. This could possibly be because they were in a hurry to get back to Andy, or because they were no longer afraid of him. He seems to have become more well-adjusted, happily rocking out to his music and seeming comfortable with his job.