Slade shared the award with Wolves chief Kenny Jackett after guiding the unfancied O's to third in the table.
"It's nice to get, particularly when it comes from other managers in your league and I was very thrilled to get it," he said.
"I won it once before with Yeovil and to get it again with Leyton Orient is very satisfying. But it's not just down to me, it's down to the players and the staff.
"There have been some very good performances by managers in our league; Shaun Derry keeping Notts County up; Steven Pressley at Coventry with a 10-point deduction and playing at Sixfields; Nigel Clough taking Sheffield United to the semi-final of the FA Cup and from bottom to seventh.
"It's been a remarkable season so for me to get that award was a very proud moment."
Orient are now just one game from following title-winners Wolves into the Championship after reaching the play-off final by beating Peterborough 3-2 on aggregate.