The 39-year-old Surrey batsman, who quit Test cricket in 2015, is fifth in the list of all-time Test run scorers.
"You try to fight the inevitable but you need to get out while you're ahead," he told BBC Sport.
"It's the last time I'll play a four-day game here [at Lord's]. I'll be 40 in a few months, this is about the end of my time in county cricket."
Sangakkara has contracts to honour in Twenty20 competitions taking him into 2018, but added: "My career might have a few more months [left] but that's about it."
He averaged more than 57 runs across 134 Tests, making 11 double centuries in that time, and joined Surrey for the 2015 season.
He scored more than 1,000 first-class runs last season but, despite hitting two centuries against Middlesex over the weekend, Sangakkara believes September is the right time to end his career in the longer format.
"The biggest mistake that sometimes you can make is that you think you're better than you really are," he said.
"Cricketers, or any sort of sportsperson, have an expiry date and you need to walk away.
"I have been very lucky to play for as long as I did so but there's a lot more life to be lived away from the game."
(Excerpts from BBC News)