Dennis began his career in the academy at Premier League club Arsenal, before being released and signing a three-month contract with Second Division club Brentford during the 1996 off-season. He made the first senior appearance of his career as a 75th-minute substitute for Carl Asaba in a 2–0 defeat to Crewe Alexandra on 12 October 1996. Dennis broke into the team during the latter part of the 1996–97 season and made one appearance in the Bees' unsuccessful Football League play-offs|play-off campaign]. He made just six appearances during 1997–98 and after the club's relegation to the Third Division at the end of the season, he signed a new one-year contract. Dennis was an unused substitute on one occasion during the 1998–99 season and was released in May 1999. Dennis made 21 appearances and scored no goals during three seasons at Griffin Park.
In February 1994, Dennis was convicted of manslaughter together with his brother, after the duo stabbed 20-year-old Simon Geary to death in the Highbury neighbourhood. In 2001, he was found guilty of the 1999 murder of 23-year-old Batatunde Oba in a fight at an Ealing nightspot. The conviction was later overturned on appeal.