Defoe's calm 60th minute finish, his third goal in two games for his new club, took them to a 2-0 start for the first time in franchise history.
The former Spurs striker scored both Toronto goals in a 2-1 win over Seattle in their first game of the season and he continued his fine form on his home debut at a packed BMO Field.
Defoe, who hopes to stay in the hunt for an England World Cup place, could well have scored more goals but for DC United keeper Bill Hamid.
Hamid made a fine save to deny him in the 20th minute and Defoe saw a shot rebound off the post four minutes later.
But he finally beat Hamid on the hour with a goal made and finished by Toronto's big name players.
United States midfielder Michael Bradley played in Brazilian veteran Gilberto and his fierce shot rebounded to Defoe, who calmly slotted home to show why the Canadian side splashed out $100m to sign the experienced trio this winter.
Dwayne De Rosario could have put the game out of reach in the 76th minute after Defoe played him in on goal, but he couldn't beat Hamid from a tight angle.
Meanwhile, Robbie Keane scored late in the first half as the Los Angeles Galaxy bounced back from a season-opening defeat with a 1-1 draw against Real Salt Lake.
Alvaro Saborio had earlier put Salt Lake ahead in the 19th minute.