Sid Nelson was a League Two play-off winner over the weekend – his mid-season loan switch coming up trumps.
Millwall announced recently that the Lewisham-born centre-back, 23, was being released this summer.
Nelson spent the first half of the season on loan at Swindon Town but headed to Tranmere Rovers in the January window. He played the full 120 minutes at Wembley on Saturday with Connor Jennings scoring just before the tie against Newport County would have gone to penalties.
Tranmere are back in League One for the first time since 2014 and have enjoyed back-to-back promotions.
Rovers boss Micky Mellon said: “We showed footballing ability, togetherness and fight. To see ourselves through was terrific.”
It is a first piece of silverware for Nelson, who has previously had loans with Newport, Yeovil and Chesterfield.
The South Londoner was handed his Lions debut by Ian Holloway in the 2014-15 Championship season.
He featured 38 times for Millwall but had not had a first-team look-in since they got promoted to the Championship.
Tranmere were due to hold a civic reception and celebration yesterday.