The 18-year-old quick bowler has agreed a 12-month contract for the 2020 campaign.
Telford-born Stanley impressed academy coach Elliot Wilson with his character and attitude and was then retained as part of the 2018-2019 intake.
He played for the Seconds at the end of 2019 campaign in two T20 matches against Gloucestershire at New Road and a three-day match against Glamorgan at Monmouth School.
Stanley played age group cricket for Shropshire and turns out for Shifnal in the Birmingham Premier League.
Wilson said: “Mitch has had a torrid summer-and-a-half with injuries – which aren’t uncommon for fast bowlers; a real persistent elbow problem in 2018 and then an issue with his side and hip – during the first part of last season.
“But, when we got him right at the back end of last summer, he proved to be incredibly impressive.
“With what he has done during the winter months and then his performances at the back end of last summer, he thoroughly deserves the opportunity the club are going to give him over the next 12 months via the contract.
“He did as well as anyone has done in the Under-17s and Academy and I was as impressed with him as I was with Josh Tongue, Adam Finch and Dillon Pennington.”
Worcestershire Cricket steering group chairman, Paul Pridgeon, said: “Mitchell Stanley has signed a one year rookie contract. He is a fast bowler who bats a bit as well.
“He has shown a lot of promise and, of all the Academy lads, Alex Gidman (First Team Coach) and Alan Richardson (Bowling Coach) have been most impressed with him.
“He played a couple of Seconds games at the end of last season and is a big lad like Dillon Pennington, Adam Finch and Josh Tongue who gets the ball down from a good height.”