Di Mario was born in Rome. She started her sport career as a swimmer, moving to water polo aged 15. After playing for Vis Nova Roma, in 1997-1998 she moved to Orizzonte Catania for which, As of 2015[update], she is still playing. With Catania she won a total of fifteen Italian national titles, five European Champions Cups and one European Super Cup.
She debuted with Italian national team in 1999 in the European Games held in Prato, where Italy won the gold medal. With Italy she also won the European gold medal at Ljubljana in 2003 and at Eindhoven in 2012 (MVP) and two silver medals at Budapest in 2001 and Belgrade in 2006. Di Mario took part to five world championships: in 2001 at Fukuoka, at Barcelona in 2003, Montreal in 2005 (top scorer), Melbourne in 2007, Rome in 2009, Barcelona in 2013 and Kazan in 2015 (winning a total of one gold, one silver medala and one bronze), after which she left the national team. At the 2004 Summer Olympics she was part of the team winning the gold medal. She returned playing for Italy's national team in 2012.