Was Sgt. Pepper the first Prog Album?

Prog is not 1 genre, is many, sounds to me more relevant to look for early prog albums within each genre 

You have to do a lot of investigation/research into the history of other bands and sum it up for yourself. You have to observe any change of band members and also taking notice of exactly what specific time period a particular band changed to progressive in the mid or late 60's. Piper At the Gates Of Dawn should at least be considered one of the first prog albums. I believe the album created "Space Rock". The songs were sometimes like "insane children's songs"  and it contained space rock jams. I'm not sure what year it was released but "Music In A  Doll's House" by Family was very different. I would still have to speculate that the Beatles influenced them all. Check to see what year Days of Future Passed was released. It's revolting to try and figure out who got there first, and if you observe what was revolving around the Beatles during that period in time you might get a "when all is said and done, eat me" kind of answer.

It pre-dates ITCOTKC and has all the classic prog attributes including mellotron, harmonies, studio pastiches, orchestral parts, odd song juxtapositions.