Heath scored in the 41st minute off a free kick in front of 9,129 fans at Sahlen Stadium. There had been questions during the week whether an injured right foot would allow her to play.
"As soon as I saw an opportunity like that, I just wanted it really badly," Heath said. "It was shot all the way, for sure."
The Flash were frustrated to not be awarded a penalty kick in the 49th minute when Portland defender Kathryn Williamson was given her first yellow card for colliding with Abby Wambach.
"Wambach's clearly in and scores off that," Flash coach Aaran Lines said. "It should've been a red card."
The red came just seven minutes later, as Williamson was ejected in the 56th minute with a second yellow card. The dismissal forced Portland to play a player short the rest of the match, but Western New York could not capitalize on the advantage.
"When Portland's down a player for 35 minutes and we're not able to put a ball in the back of the net in that time, Portland deserves it," Lines said.
Former Flash star Sinclair — a native of Burnaby, B.C. — scored in stoppage time on a pass from 72nd-minute substitute Alex Morgan, who had been held out of the starting lineup because of a knee ailment.
Sinclair headed directly to the few dozen vocal Thorns supporters stationed behind the goal to celebrate and blow kisses after effectively sealing the first year team's championship.
"I was going to go over to our bench but I thought, 'I'm not running that far,'" Sinclair said.
Playing with the advantage, the Flash attempted 18 shots, nine more than the Thorns. Portland goalkeeper Karina LeBlanc, from Maple Ridge, B.C., made seven saves in the short-handed win.
"The team is fantastic," Heath said. "You saw it in the semifinal and you saw it again here. It's almost like when some kind of adversity happens, our team comes together more."
The Flash failed in their bid to win their fourth title across four leagues in four seasons. That didn't stop goalkeeper Adrianna Franch from reflecting on the crowd.
"I had chills as soon as I walked out here for warm-ups, let alone when the game started," Franch said. "If we can continue to have that, this league is going to go far."
Heath was selected the match's most valuable player despite subbing off in the 63rd minute. She had three shots on goal in addition to her goal.
Wambach did not register a shot on goal, though she attempted three shots. Flash teammate Carli Lloyd led all players with four shots on goal