Brown delivered his decision to Aberdeen chairman Stewart Milne on Monday night, two days after the sixth goalless draw of the season at Pittodrie, against Motherwell, left the Dons in ninth place in the Clydesdale Bank Premier League with three games until the split.
Aberdeen will face Celtic at Parkhead on Saturday at least two points off the top six - Hibernian could extend the gap when they face Motherwell tomorrow night - after taking two points from their last three games.
The 72-year-old's contract expires at the end of the season but Brown insisted it was his decision alone to move aside.
Brown, whose assistant Archie Knox will leave the club in the summer, said: "This has been a delight, this job. The dressing room and the response has been exceptional, the goodwill from the fans has been consistently exceptional.
"You have a shelf life in any job. My intention was to wait until the end of the season but then I thought it was unfair to spring it upon the club.
"Archie and I still have the ambition to take this club into the top echelons of the league and we have not ruled that out, far from it.
"I wouldn't want anyone to think I am retiring because we don't think we are going into the top six.
"Two of the last three games have been very good. The quality against Kilmarnock was very good and we created four or five chances against Motherwell, the second best team in Scotland. We had a disappointment against Ross County in between."
Brown, who will join the Dons board when he leaves the dugout, added: "I made the decision to retire. There are other managers in the SPL whose contracts are up and there is no speculation about them. I think it's to do with my age but that's fine.
"I feel confident in what I have done here. I don't feel any way that I should feel embarrassed. I feel it has been a productive period for Aberdeen.
"This year we are criticised extensively for not scoring goals. No-one says we have the second best defensive record in the SPL. There is an art to defending too.
"I think the team has come good. We are two points off the top six with horrendous injuries and selling players.
"I think that has been success. We are not at the top yet but the season has not finished.
"I read that Brown is under pressure. Believe me, Brown has never been under pressure here."
The former Scotland manager believes he will leave his successor a strong legacy.
Brown took over in December 2010 with the team struggling at the foot of the table before leading them to a comfortable finish in the bottom six.
But he failed to secure top-six football in his first full season and the Dons look to be heading for a fourth consecutive bottom-half finish.
"I would say an exceptionally good squad," he said, when asked what he was handing over. "When I came here we were bottom of the league, the team had lost 12 out of 16 matches.
"We stabilised that, we have sold six players, the best player retired at the time, Paul Hartley. We had to replace these guys without a transfer budget.
"We had three semi-finals, comfortable finishes, to have had injuries, misfortune with decisions, I think you can see this squad is just about to take off."
Brown will join the Aberdeen board as a non-executive director - and thus have a say in the appointment of successor - when his management spell ends.
"If someone came and they wanted him to come in tomorrow, and he's available, I would step back immediately," he said.
"But I'm reluctant to do that because there is still work to be done this season and I'm confident we can finish the season very well."