League MVP Smith struck in the fourth minute and was in on the play that led to Portland's second score courtesy of an own goal by the Current's Addisyn Merrick shortly after halftime in the NWSL Championship match at Audi Field in Washington.
The victory makes it an NWSL record third title for the Thorns, who won the league's inaugural championship game in 2013 and lifted the trophy again in 2017.
The Current had a costly turnover before Smith went down the field, maneuvered around goalkeeper Adrianna Franch and deftly scored in the fourth minute.
Smith nearly had another goal in the 27th minute, but her shot went just wide. She is only the second MVP recipient to score in the championship game, joining Lynn Williams for North Carolina in 2016. At 22, she's also the youngest player to score in a league final.
Moments after the Merrick own goal, Morgan Weaver had a shot from distance but Franch got a hand on it before it hit the crossbar and caromed away.
The NWSL was rocked this month by the Yates report that showed systemic abuse and misconduct, spanning multiple teams, coaches and victims, with the Portland Thorns one of the franchises at the center of the investigation.
Audi Field was nearly sold out for the game -- which was broadcast in prime time on network television for the first time -- with some fans holding up a sign that read: "Support The Players."
Portland had a first-round bye in the playoffs and beat the San Diego Wave 2-1 in the semifinals to advance to the championship game.
Smith, the No. 1 pick in the 2020 NWSL draft and a member of the U.S. national team, scored 15 goals for the Thorns over the course of the regular season and playoffs and was also named MVP of the championship game.
The Current began play in 2021 as an expansion team, with many of their players coming over from the defunct Utah Royals. Last season they finished in last place in the league.
The Washington Spirit won last year's NWSL Championship.
Information from the Associated Press was used in this story.