The centre-half confirmed on his Facebook page that it was "a hard decision" as he leaves Mill Field to play in the Ryman Premier League next season for John Coventry's East Thurrock United.
Leader won the fans' player and players' player of the year awards for the previous two seasons having played 93 games for the Millers scoring 14 goals.
And the now former Miller jokingly remarked on his Facebook page that "all Ryman 1 North division strikers can officially go out and celebrate because I have finally left the division."
In his time at Aveley Leader clocked up over 100 appearances for the side and manager Justin Gardner regards him as "one of the best and most consistent players I've ever worked with."
Although he was upset at losing a player of Leader's quality, Gardner likes nothing more than to see players progress and feels that the move to East Thurrock "is a good move for him."
"I've kept trying to help Jay out and get him to move to a higher level and still feel he is better than the Ryman Prem" said Gardner.
"Last year Jay said that he didn't want to leave the club and that he just wanted to play for me, but I feel it's a good move for him. He's chosen a great club and he'll be in good hands."
Leader was almost an ever-present in Aveley's run to the play-offs in the 2012/13 season, appearing in 42 of Aveley’s 43 league games, including the play off semi-final against Maldon.
A string of top class performances that season earned Leader a place in the Ryman North team of the year and the centre-half thanked everyone at the club on his Facebook page, saying "it's been a pleasure."
Gardner had nothing but praise for the 24 year old, saying "he needs to test himself at a higher level and if he dedicates himself and commits himself I'm certain he can play professionally in the future.
"And if it doesn't work out then he is always welcome back at Mill Field and he knows where my door is. For now though I must thank him for his exceptional service to the club."
The Millers boss also commented on the conduct of Leader's new boss, John Coventry, in his handling of the transfer between the sides.
Gardner said "Covo has conducted himself perfectly and has been a man of his word.
"He always made it clear of his admiration for Jay and how much he would like him at his club, but not once made a request for him.
"Instead he just told me that if Jay was to ever want to leave let him know. He's been first class and, as I've said, he's joining a great club and will be in good hands with Covo."
Finally Gardner wanted to reassure any Aveley fans that may be concerned about losing a player of Leader's quality, saying "I want the fans to trust me and trust what I'm doing at this club.
"There will be replacements for Jay and I'm looking forward to building a new team that will challenge for promotion next season.
"We have already re-signed a number of our better players from last season and are in discussions with a number of other top players."
Aveley fans will get a chance to witness Leader playing for his new side first hand when East Thurrock take on Aveley in a pre-season tournament over the weekend on the 26th and 27th of July at Mill Field.
The tournament that will also include tenants Grays Athletic and fellow Ryman North side Romford in an Emirates Cup style tournament with each team playing two games over the two days. Full details are to be confirmed.
Everyone at the club would like to thank Jay for his service at Mill Field and wish him all the best in the future.