The Sharks appeared headed for a regulation victory when Mark Bell scored on electric passes from Jonathan Cheechoo and Thornton with 5:42 left in the third period. But Bill Guerin forced overtime with 5.6 seconds left in his St. Louis debut, tipping home Keith Tkachuk's pass after the Blues pulled goaltender Manny Legace for an extra attacker.
Brown, who returned to the Sharks this season, ended it by reaching around a defenceman and tipping home rookie Matt Carle's shot, triggering a bench-clearing celebration on the boards.
Milan Michalek scored two goals during 5-on-3 advantages on assists from Thornton, who got an early start on his defence of his MVP award and scoring championship from last season. Carle also scored for the Sharks, and Vesa Toskala recovered from two soft first-period goals to make 31 saves.
Legace made 29 saves, while Dan Hinote, Jamal Mayers and Lee Stempniak scored for the Blues, who are winless in their last nine openers - the longest streak in the NHL.
But after finishing last overall in the league with just 57 points, St. Louis clearly is markedly improved. Veterans Radek Dvorak, Guerin, Legace and Hinote debuted with St. Louis, while Martin Rucinsky, Jamie Rivers and Doug Weight officially returned to the club.
Hinote quieted the sellout Shark Tank crowd just 36 seconds in with a goal on his first shift with the Blues after six seasons in Colorado.
Michalek, the second-line wing who led the NHL with seven pre-season goals, got his first of the regular season on a sharp pass from Thornton early in the second period. Thornton then whipped a pass off Michalek's stick and into the net before Michalek even realized he was open, putting San Jose ahead 3-2.
But Stempniak tied it seven minutes later, adeptly deflecting Guerin's shot from the high slot.
Referee Rob Martell left the game after being hit in the left eye by one of Michalek's shots during a power play in the third period. The game also was delayed 18 minutes to fix problems with the boards later in the period.
The Sharks went ahead when Cheechoo, who had two assists, adroitly poked the puck away from defenceman Eric Brewer to set up a 3-on-1 break. The Sharks' new top line exchanged criss-cross passes before Bell buried his first goal since arriving in an offseason trade from Chicago.
Notes: Blues owner Dave Checketts, general manager Larry Pleau and new team president John Davidson watched the game together in the press box. ... The Blues begin the season with a three-game California road swing before their home opener Oct. 12 against Boston. ... Rookie D Marc-Edouard Vlasic got a high-pressure assignment in his NHL debut, their 5-on-3 penalty-killing team late in the second period. Vlasic held his own as San Jose killed the Blues' two-man advantage for 107 seconds, eliciting a standing ovation.