The Jamaican, who had looked in great shape in the morning heats, was never at the races after hitting the first hurdle and clattered several more to round off a disappointing defence of her title.
Harrison set the world record of 12.20 at the London Stadium past year, but only after missing out on the United States team for Rio, and suffered more disappointment here as she clipped hurdle after hurdle to come home fourth in 12.74.
"When I knew I felt that I knew it was in me to do it".
Pearson's winning time was actually slower than in her semifinal, in which she breezed to victory in a 12.53, though it seems unlikely that the two-time World champion will be too bothered as she capped off her return from injury with one of the sport's biggest prizes.
Farah, who won the 10,000m once again last weekend, dug in to finish 0.43secs behind new world champion Edris, while Paul Chelimo of the United States claimed bronze.
"If I don't feel that spark I know there's something wrong but I felt it straight away".
Pearson split acrimoniously from her longtime coach Sharon Hannan after finishing second behind Brianna Rollins at the 2013 world titles.
It proved to be an inspired decision.
"But the emotion just escaped my body because I was so excited and so happy to have achieved what I have worked so hard for". It was like Beijing in 2008 (she won a surprise Olympic silver) when I didn't come in as one of the favourites.
Pearson, whose English mother brought her up as a single parent and in very straitened circumstances, taking two jobs so her daughter could realise her dream, said it had been a long and hard road back to the top.
"Silver tastes like gold tonight", she said.
"At the end, I could see Sally had won and I thought "it's me and Sally again".