McNamara, who has also refereed in the EFL, will be banned from Monday, November 26 to December 16.
"The FA can confirm that referee David McNamara has been suspended for 21 days, starting from Monday 26 November, after accepting a charge of 'not acting in the best interests of the game'," an FA spokesperson is quoted as saying.
"This follows an incident in the FA WSL match between Manchester City and Reading on Friday 26 October when he failed to determine which team would kick off the match by the toss of a coin, as required by the Laws of the Game."