The O's won four of their seven fixtures in the month and lost just once, a 2-1 away defeat to Scunthorpe on March 29.
Dean Smith of Walsall and Portsmouth's Guy Whittingham were also in contention for the award but Slade has received recognition for a run that propelled his side into the top half of the table. It is the first time he has been received the prize since taking over at Brisbane Road in April 2010.
Slade told the club's official website: "It's nice to be recognised, it's not just me it's the management team as a whole really, it's a team award. We had a reasonable month, albeit we lost the very last game in March but it was a satisfactory month and we just want to finish the season off in style.
"We had some good performances particularly at home and it gave us some momentum to keep the season going and that was important to us.
"March in many ways seems a long time ago though, as they say a week in football is a long time and we've had mixed fortunes since then to be honest, plus we've had problems in terms of our personnel in terms of getting them fit and ready and on the pitch."