The 23-year-old, who also won the 2018 Ginetta Junior and 2021 GT4 Supercup titles with Elite Motorsport, will partner Shaun Balfe after the Am driver moved across from Barwell Motorsport.
Smalley was selected as Porsche's junior driver for the 2022-23 season as he went on to finish runner-up in the Carrera Cup in 2022, before winning the title the following season with Team Parker Racing.
“I’ve always wanted to do GT racing and I think this is a fantastic step to becoming a factory driver,” said Smalley.
“I looked at all the options – being the Porsche Junior has been an incredible experience and winning the championship was great.
"This just seemed like the best next step for my career. The way the world is now with GT racing, it’s all on the up and there’s a lot of opportunities.”
Despite being new to the championship, Smalley is aiming for outright success alongside Balfe, who is one of the most experienced Ams in a series he first contested in 1999.
Last season, Balfe drove a Lamborghini Huracan Evo GT3 alongside factory driver Sandy Mitchell at Barwell, with the pair finishing fourth overall in the standings having won at Snetterton and Donington Park.
Balfe was the first 720S customer in British GT in 2019, and finished five points shy of the title with Rob Bell despite missing the first two races after teething troubles on his family-run car at Oulton Park.
He was also a race-winner with Audi R8 LMS Evo II in 2022 alongside Adam Carroll.
“It should be an exciting season,” added Smalley. “The goal is to win the title – the approach doesn’t change to last year.
“But I’m also realistic and I know it’s going to be very difficult – there are some of the world’s best drivers on the grid this year and it’s an incredibly tough challenge.”
The second Garage 59 McLaren will be driven by Marcus Clutton and Morgan Tillbrook, who remain together for a fourth season.
The pair, who have taken three outright British GT wins aboard McLaren machinery, ran their own Enduro Motorsport team over the past three years, but disbanded the squad at the end of 2023 in order to focus on their racing.
“We did a two-day test last week, which was positive, the first time with the team,” said Clutton.
“For me personally, the stress levels have come down massively and I think it’s potentially the final ingredient for that big push now. I think, if there’s ever a time where we’re ready, it’s now.”
