It's the second year in a row that a freshly delisted AFL player has won the award, after former Adelaide player Brad Symes won it while playing for Central District in 2012.
Thomas enjoyed a sensational season for Norwood, winning the prestigious individual honour with 26 votes ahead of Port Adelaide small forward Sam Gray with 21.
He is the 11th Norwood player to win the award and the first since Andrew Jarman in 1997.
The 26-year-old played four games for the Power this year, but was one of seven players delisted on Monday.
He likened the low of losing his place at the Power and then the high of Tuesday night's medal win to an amusement park ride.
"I'm a bit overwhelmed at the moment – it's been a rollercoaster 24 hours," Thomas said.
"To have a sweetener like that heading into a really important month of footy is just really great.
"To be alongside those [previous winners] is something that I'll obviously hold pretty special and dear to my heart."
Finding himself eight votes behind Redlegs teammate and Crows-listed Mitch Grigg after round 14, Thomas admitted he thought he was too far back to win.
"My girlfriend Stacy and I said at round 14 we thought we'd probably done our dash, then all of a sudden some of the votes started rolling through.
"I'm still so surprised."
Norwood will play West Adelaide in the second semi-final on Sunday, with a grand final place at stake.
"I haven't played finals at Norwood, I haven't played finals in Adelaide so my attention turns to that and it's pretty exciting to think of the possibilities," Thomas said.
"We've had such an outstanding year as a team and I'm looking to giving them some love on the weekend and hopefully coming away with some wins."
Thomas attracted interest during last season's trade period from Richmond but remained with Port Adelaide, the club he debuted with in 2006.
Meanwhile East Fremantle midfielder Rory O'Brien streeted the field to win Monday night's Sandover Medal for the WAFL's best and fairest player after polling 55 votes.
Of interest to AFL draft watchers was the performance of the Swan Districts forward Dayle Garlett, who polled 24 votes to finish in the top 10.
The talented Garlett was overlooked in last year's NAB AFL Draft because of perceived off-field issues, and also missed selection in the NAB AFL Pre-season Draft despite training with Essendon over the summer.
He remains of interest to AFL clubs, with several clubs meeting the 19-year-old during the year.