Cassola was one of the co-founders in 1989 of Democratic Alternative, the green party in Malta. Between 1990 and 1997, he was the Party's delegate to the European Green Party (EGP). He unsuccessfully contested all national elections with the Party since 1992. Between 1994 and 1997, he served as local councillor in Swieqi. His best result in Maltese politics was in the 2004 election for the European Parliament when he obtained 9.33% (or 23,000 votes) of the first-preference votes, just failing to be elected one of Malta's five MEPs.