"We cannot go on as we have done before. I have been very close to resigning several times in the past but I have stayed on for the players' sake," the coach told Swedish media after a two-year stint in charge.
In his resignation letter, the Swedish coach accused the Sierra Leone federation of failing to organise preparatory internationals and of changing the squad he selected for last week's qualifier in Tunisia, which Sierra Leone lost 2-1 to fall five points behind them at the halfway stage of the group phase of the preliminaries.
"No national coach in the world would accept to be treated like this. Nobody communicated with me - so I do feel you understand my choice to withdraw from all missions with the Sierra Leone national team," the letter concludes.
Mattsson took over as coach in 2011 and took Sierra Leone to the brink of qualifying for the 2013 African Nations Cup, losing out only on the away goals rule.