Pearson has previous managerial experience from a stint in charge of Carlisle United, while he has also been assistant manager at Stoke City and West Bromwich Albion.
Saints chairman Leon Crouch told the club's official website: "We are very grateful to John and Jason for all their hard work and their passion after taking over in very difficult circumstances.
"However, it was clear that the uncertainty of the management situation was having a detrimental effect on the players.
"Nigel is a top-quality coach with plenty of experience and good contacts in the game. He is also a disciplinarian with a strong work ethic and I believe he will inject new impetus into our campaign.
"I believe he is the right man to steady the ship and get us moving up the table once more. He is a winner and he will instil that mentality into the players."