Created by Chris Claremont and Marc Silvestri, Mr Sinister (real name Nathaniel Essex) was first introduced in Uncanny X-men 221.
Claremont’s original vision was vastly different to how the character eventually turned out. Here’s Claremont on how he envisioned it.
"Dave Cockrum and I were over ideas and what we were coming towards was a mysterious young boy - apparently an 11-year-old - at the orphanage where Scott (Cyclops) was raised, who turned out to be the secret master of the place. In effect what we were setting up was a guy who was aging over a lifespan of roughly a thousand years. Even though he looked like an 11-year-old, he'd actually been alive since the mid-century at this point - he was actually about 50. He had all the grown up urges. He's growing up in his mind but his body isn't capable of handling it, which makes him quite cranky. And, of course, looking like an 11-year-old, who'd take him seriously in the criminal community? So he built himself an agent in a sense, which was Mr. Sinister, that was, in effect, the rationale behind Sinister's rather - for want of a better word - childish or kid-like appearance. The costume... the look... the face... it's what would scare a child. Even when he was designed, he wasn't what you'd expect in a guy like that."
You can see the fruition of Claremont’s Sinister in X-men forever at the moment. This thread focuses on the 616 version.