Two goals from a revitalised Erik Lamela gave Spurs a comfortable lead at half-time but Mauricio Pochettino made 10 changes at half-time, replacing every outfield player, and the MLS side subsequently fought back in the second half, as strikes from Andrew Wiedeman and youngster Jordan Hamilton drew Toronto level.
But Spurs would not be denied and Lamela's replacement Townsend – hopeful of once again proving his worth this season following an injury-disrupted 2013-14 – struck in the 85th minute to earn Pochettino's men a hard-fought victory.
Spurs dominated from the outset, controlling possession and made use of their time on the ball when Lamela first struck in the 16th minute.
Christian Eriksen initiated the foray forward, linking with Roberto Soldado, who laid the ball off to the former Roma man, who tucked away a left-footed shot.
Ex-Spurs star Jermain Defoe - leading Toronto's forward line - then beat Spurs' defensive line in the 19th minute, but he was unable to score against his former side as goalkeeper Brad Friedel came off his line to collect.
Lamela doubled Spurs' lead with a near-identical effort in the 40th minute, having begun the move himself.
The 22-year-old found Aaron Lennon in midfield, and the winger subsequently picked out Soldado, who laid the ball on a plate for Lamela to finish from just inside the box.
Tottenham manager Mauricio Pochettino substituted every outfield player at half-time, including club debutant Ben Davies, who impressed at left-back after sealing his move from Swansea earlier in the day, while Toronto coach Ryan Nelsen made seven changes.
Wiedeman then pulled a goal back for Toronto in the 65th minute, having been released by Daniel Lovitz following a fine tackle in midfield.
And Toronto were soon level, Hamilton finishing off a fine move with a clever finish from close-range.
But Townsend ensured Tottenham would leave victorious, with a curling finish into the top-right corner as the clock ticked down.
Spurs' win marks the first victory of their tour following a 3-3 draw with Seattle Sounders on Saturday.