Horner played at Middlesbrough from 1960 to 1969, primarily as a defender. He moved on to Darlington where he played until 1975.
He was player-manager of Darlington during the 1974–75 season, in which the club had to apply for re-election. He left the club at the end of the season and was appointed as coach to Darlington's arch-rivals Hartlepool United. In October 1976, he took over as manager, and had two separate spells in this role: one from 1976–1983 and the other from 1983-1986. Although Hartlepool applied for re-election to the League on three occasions, Horner also guided the club to a respectable 9th-place finish in 1980–81, during which the club had been promotion contenders. In his second spell at the club, Hartlepool finished 7th in 1985–86, in which they had also been a promotion contender for much of the season.