Former Scarborough FC man Sinnott and ex-Boro assistant manager Reed had pushed Gainsborough into the top few of the Evo-Stik Premier, but in recent weeks that have struggled to put any positive results together.
Their last two games have seen them slump to heavy defeats at the hands of rivals Warrington Town and Farsley Celtic.
Gainsborough have moved to put Liam King in charge on a temporary basis.
Prior to his departure Sinnott admitted he was feeling the pressure.
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He said after last night's defeat: "Any manager, even if they are up at the top in a two or three horse race, that says they don’t feel the pressure for me is telling fibs.
“It comes with having to win. It’s not nice. It’s a horrible feeling. We have to do something to rectify it.
“You always think about your own position, even when you are doing well, if that makes sense. There are two sides to a coin.
“As a game it’s a lot more analysed now, even at non-league level without being disrespectful. The analysis from the top echelons has filtered through.
“So these questions will be asked and they are understandable."