Adrian is one of many wingers (Mark Barham, Louie Donowa, Dale Gordon, Ruel Fox, Darren Eadie, Keith O'Neill) that have been produced by the club's youth team setup. Adrian made his debut in the 1-0 victory over Wolves in August 1996 and made a total of 11 appearances by the end of that season. He had made 121 appearances by the end of his spell at Carrow Road although over 40% of them were as substitute.

Adrian was placed on the transfer list in mid August 1999 along with Darren Kenton. Although he signed a new contract in March 2001, Forbes was surprisingly transferred to Luton on 16th July 2001 for £60,000. With Nigel Worthington unable to guarantee him first team football next season, Forbes obviously saw stepping down by two footballing tiers as the lesser of two evils.After a successful 2001/02 season that saw Luton promoted from Division Three, an injury sustained during the 2002 pre-season means that Adrian will miss most of the 2002/03 season. 2003/4 was more profitable as he scored 14 goals for the Hatters

Failure to agree a new contract with Luton at the end of the 2003/4 season saw Adrain depart to Wales, joining Swansea City on a two-year contract.

The Swans released Adrian on 30th May 2006 due to the form of Leon Brittain and youngster Shaun McDonald. Adrian then joined Blackpool on 23rd June 2006

After making 36 appearances, scoring once, for Blackpool, Adrian transferred to Millwall on an eighteen month contract on 3rd January 2008