Alan was born in Leighton Buzzard and began his career with Luton Town’s youth team before joining Cambridge United in 1975. He became a prolific goalscorer for Cambridge which saw him net 82 goals in 185 matches which brought the attention of bigger clubs. First Division Derby County signed him in 1979 and he took to the top-flight well scoring nine goals in 18 matches but Derby were relegated. He remained with the “Rams” for the 1980–81 season scoring 10 goals as they finished in 6th position. Alan was then signed by Everton but he had a frustrating spell at Goodison Park scoring just three goals in 21 and left before the end of the 1981–82 for Stoke City. He played eight matches for Stoke scoring once which came in a 4–3 defeat away at Southampton.
Biley then signed for Bobby Campbell’s Portsmouth in 1982. He became a hugely popular figure at Fratton Park helping the South Coast side to the Third Division title in 1982–83. After scoring 65 goals in 119 games for Pompey he left for Brighton & Hove Albion where he spent two seasons and then a short return to Cambridge United. After a brief spell at Panionios he ended his career with a brief spell at Waterford United where he was signed by ex team mate Andy King. Alan made his League of Ireland debut on the 15th of January 1989 at Cork City but only made one other appearance for Waterford.