The Strahd doll was first discovered by the waffle crew near Krezk. Soon after, Strahd actually spoke through the doll to Paultin. Strahd urged Paultin to betray his friends and to come to Castle Ravenloft.
When the heroes were resting in Van Richten's tower, he told them he knew of a ritual that the group could use that would trap Strahd in the doll. 
The heroes eventually used the doll in a ritual to trap Strahd inside of it.
The waffle crew gave the Strahd doll to Drizzt Do'Urden for safekeeping. He brought it to Gauntlgrym.
Izek brought the doll from Gauntlgrym to the Shadowfell and gave it to Strix under orders from Lady Wachter.  Soon after, Marisiana nearly killed Strix and stole the doll from her. 
Diath found the outer skin of the doll in the home of Esvele Rosznar in the city of Waterdeep. He soon learned that this meant that Strahd had somehow been freed.
The Real Strahd Doll
Holly actually made a real Strahd doll, which is an accurate representation of what the doll looks like in-game. It is also known as "Mr. Shambleface."