'Twas Steve's idea.

" The first story is nothing great, but perhaps we can make something of him.
'Twas Steve's idea. "

“

In Search Of Steve Ditko
(2007)

Fantastic Four Documentary

(Jack Kirby art)

Stan Lee is quoted in 1963 as saying this:

"Steve Ditko is gonna draw him. Sort of a black magic theme. The first story is nothing great, but perhaps we can make something of him—'twas Steve's idea, and I figgered we'd give it a chance, although again, we had to rush the first one too much."

The first appearance of Doctor Strange was Strange Tales #110 as just a five page filler. 

As editor in chief Stan would have had a say in any new character's development but he didn't co-create the character by any means.