He becomes a consumer PR strategist and will work closely with fellow ex-Sun man Chris Pharo, who in 25 years at the tabloid rose to head of news and then associate editor.
Larcombe and Pharo were among a group of journalists tried for alleged illegal payments to public officials, in the Metropolitan Police's controversial Operation Elveden, which lasted more than three years. Both were cleared by a jury at the Old Bailey in October 2015.
Following their acquital, Pharo joined 72Point as chief operations officer in 2016, and is now MD.
Larcombe, who was at The Sun for 13 years, moved to Kent full-service agency Zest. He has also been a royal pundit for ABC Network America and is the author of a biography entitled Harry: The Inside Story.
Larcombe said: "72Point are one of the fastest growing PR agencies in the country. I'm looking forward to joining this experienced team as they continue to shake up the industry."
72Point now employes 48 staff, while sister business SWNS, a news agency, has around 100.