Josh Ford’s stoppage-time penalty secured victory for 10-man Holbeach United.
He put the Tigers ahead in the first half but Ollie Pinner’s dismissal for a professional foul led to the equaliser from the spot.
However, a foul on Tommy Treacher brought a red card for Rothwell Corinthians’ keeper and Ford slotted in.
Martyn Cundy, who ended his spell as caretaker-boss, said: “In the first half we were in control of the game.
“We were winning pretty comfortably and we should have got a couple of goals.
“The next goal was going to be important but we went down to 10 men and they equalised.
“For the next 10 minutes we had our backs against the wall until we got our shape.
“Rick Drury made a couple of good saves to keep it level and then we gambled a bit by pushing for the winner.
“Fortunately for us, we got another goal at the end. It was great play by Treacher from a lofted ball as he turned past a defender and got brought down by the keeper.”
After the game, new Tigers boss Seb Hayes met the squad and Cundy believes they can be successful together.
I’ve told Seb that he has inherited a top-four side at one of the best clubs in the UCL. There are foundations in place at Holbeach and I’m sure they can win trophies.Martyn Cundy
He added: “He was only watching the game and he did not interfere at all.
“He asked me to be part of his management team but I need to spend more time with my family.
“I’m happy to help out if they need me but the plan is to step away now.
“The chairman and the players wanted me to stay but I’m glad the club have found somebody to take the job on a project for three or four years.
“Seb is buzzing for it and he’s 100 per cent committed.
“It was never in my plans to become a manager in the next couple of years.
“I will enjoy having these Saturday afternoons free to spend with the family, to be honest!
“I’ve told Seb that he has inherited a top-four side at one of the best clubs in the UCL. There are foundations in place at Holbeach and I’m sure they can win trophies.”
Holbeach: Drury, Walton, Shepherd, Tinkler, Allen, Muldrew, Treacher, Eyett, Smith, Ford, Pinner.