"It is unreal," said Southland manager Corey Finn.
"Southland has never won this so it's really special for Southland golf and really special for all these boys. I am really proud of them.
"We've got some key players - Matt Tautari - we call him the rock at the top - and he was phenomenal all week for us getting the momentum going.
"But I am really proud of all the boys - they stood up in the big moments.
Bay of Plenty manager Aaron Walsh took little consolation in the fact the Bay were the most dominant throughout the week on the East Coast.
"We're gutted," he said. "We really wanted to win this one but I couldn't be more proud of the boys' effort. We were just beaten by the better team on the day. Southland played some fantastic golf this afternoon and fully deserved their win."
McCall had a stellar past two days where he came back from two down against Tasman's Blair Riordan to halve and help Southland progress, beat Auckland number one Ryan Fox in the semifinal and Kendall in the final.
"I had pretty tough match-ups throughout the week and in all of my matches I didn't start too well but I got the putter going late in my rounds and managed the wins," said the 20-year-old from the Gore Golf Club.
"The whole team is stoked. It was what we came here to do and we are so pleased that the result came our way."
McCall changed his putter with four rounds remaining and it made all the difference.
Southland joins North Harbour and Tasman as one-time winners of the event while Bay of Plenty remain in fifth position on the all-time wins list with six behind Waikato (7), Canterbury (9), Auckland (12) and Wellington (13).
Toro Men's interprovincial final score:
Southland 3 Bay of Plenty 2
Vaughan McCall bt Brad Kendall 2/1, Cody Harper bt Landyn Edwards 2/1, Liam Balneaves lost to James Hamilton 2/1, Scott Hellier lost to Sam Davis 1 down, Matt Tautari bt Craig Hamilton 3/1
Toro Men's Interprovincial - semifinal scores
Bay of Plenty 3 1/2 Canterbury 1 1/2
Brad Kendall bt Keelan Kilpatrick 5/4, Landyn Edwards halved with Jordan Bakermans, James Hamilton bt Tom Turner 4/3, Sam Davis bt Mark O'Malley 4/3, Craig Hamilton lost to Harry Bateman 3/2
Southland 3 1/2 Auckland 1 1/2
Vaughan McCall bt Ryan Fox 4/3, Cody Harper lost to Fraser Wilkin 2/1, Liam Balneaves halved with Sam An, Scott Hellier bt Jonathan Ratcliffe 1up, Matt Tautari bt Tae Koh 1up