The Bulldogs trailed only briefly in the early stages and resisted repeated challenges from their opponents late in a 14.13 (97) to 13.15 (93) victory at Marvel Stadium on Saturday night.
A gripping contest hung in the balance until the final minute when Magpies ruckman Darcy Cameron hurried a high kick forward.
Sellwood, a mid-season rookie selection last year who debuted in opening round, traced it back with the flight of the ball and threw himself into a pack to take a brilliant mark.
"I guess I just saw the ball and was like, 'I've really got to go here, and if I go, I've got to mark it'," Sellwood told broadcaster Kayo Sports post-match.
"Luckily it stuck because (Patrick) Lipinski was probably at the drop of the ball."
Ruck star Tim English, on return from concussion, followed up by marking Sellwood's clearing kick at half-back and the Bulldogs hung on.
Matt Kennedy (29 touches), Ed Richards (24), Ryley Sanders (28) and Marcus Bontempelli (22) all had influence at different stages.
First-year key forward Will Lewis kicked three crucial goals, including the Dogs' last of the match, while Richards, English and Lachlan McNeill kicked two each.
Dogs coach Luke Beveridge was full of praise for 22-year-old defender Sellwood.
"It was a big mark. You're hoping for a big spoil, but when he comes down with it, it's pretty pivotal in the end," Beveridge said.
"He's a very courageous player, as you can see, and there's no hesitation."
English was an important contributor, helping the Bulldogs dominate centre clearances (19-10) and the inside-50 count (62-47).
The result lifted the Bulldogs (7-5) to seventh and left Collingwood (5-6-1) in danger of slipping out of the top 10 by the end of the round.
The Magpies' loss came despite the efforts of explosive midfielder Jordan De Goey, who was best afield with 30 disposals, 19 score involvements and one goal.
Nick Daicos (37 touches) and veteran Scott Pendlebury (33) - one week after breaking the all-time AFL/VFL games record - were also strong contributors for the Pies.
Tim Membrey kicked three goals, with Lachie Schultz and Angus Anderson adding two each.
Collingwood coach Craig McRae lamented his side's inability to compete at clearances until the final term, when they also freed up their ball movement and had chances to pinch victory.
"There was some really good and some really bad tonight, and we sort of lived in between that," McRae said.
"There was 62 inside-50s against - that's going to make it hard.
"Our backs defend the game as good as anyone in the competition, but we just don't want them to have to as much as they have tonight and in recent times."
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