Born in Salerno, Grassadonia started his playing career by growing from the youth ranks of hometown club Salernitana and then making his senior debut in 1988.
In November 2009 he replaced Marco Cari as head coach, being however sacked later in March 2010 due to poor results.
In August 2015 he accepted to return to Paganese, guiding the small Campanian club to a surprising qualification to the promotion playoffs during the 2016–17 Lega Pro season.
In June 2017 he was named new head coach of Serie B club Pro Vercelli, replacing Moreno Longo.
[11] On 14 February 2021 he was named new head coach of bottom-placed Serie B club Pescara, signing a contract until the end of the season.