His international racing career was piecemeal for a few years until he got the chance to race a full series in the Japanese Formula 3 championship, which he won last year.
Winning that title would have gone a long way to helping the 21-year-old secure the drive with Japanese Tachi Oiwa Motor Sport (Tom's).
"Winning the Japanese F3 championship had a massive influence on my career and was the perfect way to reward Toyota for the support they put into me last year," said Cassidy from Japan after the announcement.
"To then achieve the dream of securing a factory role with Lexus for this year was something I wasn't even sure could happen after just one year in Japan.
"Toyota evaluated me in the Super Formula testing at the end of the [F3] season, which was my first time in the bigger single seater, and I was able to top the rookie time sheets and go faster than my teammate.
"It was the perfect outcome for me and helped secure this drive."
Often regarded as one of fastest touring car series in the world, Super GT has two categories (GT500 and GT300) that compete in the same race.
Cassidy is taking on the drivers in the top GT500 class, which is the showcase for the high performance cars manufactured by Lexus, Honda and Nissan.
It'll be no easy task for the young Kiwi, as the series attracts a number of quality drivers from Europe.
This won't be a distraction for Cassidy, who has already taken on the best young international drivers during his stint in the TRS championship.
Race distances vary between 300km and 1000km with drivers in the top class having to negotiate past the slower GT300 cars.
Cassidy joins former GT500 champion Daisuke Ito in sharing the driving over the eight-round series.
Japanese Super GT
April 9-10: Okayama Intl. Circuit
May 3-4: Fuji Speedway
May 21-22: Autopolis
July 23-24: Sportsland
August 6-7: Fuji Speedway
August 27-28: Suzuka Circuit
October 8-9: Chang International Circuit
November 12-13: Twin Ring Motegi