I've been watching Danger: Diabolik (1968) and when I read the Wikipedia article I realise it's actually an adaptation of a comic book character called Diabolik which made it one of the earliest comic adaptation I had heard of.
After more research I found this article which claimed the oldest was Adventures of Captain Marvel (1941), however there are some I found which are even older, like Flash Gordon (1936).
Can anyone provide an authoritative answer as to what the first feature-length (more than 40 minutes) film adaptation of a comic was?