Dandelion Racing driver Yamamoto edged main title rival Ryo Hirakawa by two points with a fifth-place finish in Sunday's Fuji Speedway decider, earning his third title in the series.
Outgoing champion Cassidy, who needed to win the race to have any realistic chance of defending his 2019 crown, finished one place ahead of Yamamoto in fourth after coming out on top in a late battle with the Japanese driver.
Nick Cassidy（VANTELIN TEAM TOM’S）
Cassidy's tenure in Super Formula ended with a charge from the back of the grid - the result of a track limits infringement in qualifying - to fourth place.
That left him tied for third in the standings with Fuji race winner Sho Tsuboi, albeit behind on countback with one win this year to Tsuboi's two.
By leaving his pitstop until lap 30 of 40, 10 laps later than any other driver, Cassidy enjoyed a spell out front and was pulling away from the eventual winner by around half a second a lap until Tomoki Nojiri's puncture prompted his TOM'S team to bring him into the pits.
"It was very difficult for tyre temperature today, even with the tyre warmers on the first lap it was difficult, but I made some nice moves," reflected Cassidy. "I had really good speed.
When I saw people pitting, I had clean air and my tyres didn’t drop so I could keep going. So that was nice."
Cassidy bows out of Super Formula with three race wins and three poles to his credit from 27 starts, and as well as his own drivers' title in 2019 he also contributed to two teams' titles - with Kondo Racing in 2018 and TOM'S this year.
Nick Cassidy（VANTELIN TEAM TOM'S）