Nicknamed 'Reg'[1] due to his namesake Reg Grundy, he was elevated from the Sydney Swans rookie list in 2005 after playing senior football with South Australian National Football League (SANFL) club Norwood and being an All-Australian under-18s player in 2004.

Grundy's ability to outmark his opponent (usually larger in weight and height) and find the ball more frequently, have been his most notable improvements.

In 2010, after Craig Bolton sustained a season-ending injury and then later on when Lewis Roberts-Thomson injured his hamstring, Grundy was called on by Roos to play on the competition's best forwards.

He marked players including Nick Riewoldt, Matthew Pavlich, Jonathan Brown, Barry Hall and Lance Franklin.

He was forced out of the Swans' run into and through the finals due to contracting glandular fever late in the year.