He joined Middlesbrough as an apprentice – "I was in my local under eleven's side at the age of nine, and Joe Soones, the Boro scout was the first one on the scene, knocking on my door and turning up at the matches.
"[1] – and eventually made 111 appearances for the club scoring 51 league goals and helping them win promotion to the Second Division in the 1966–67 season.
After an acrimonious period at Middlesbrough, he left the club in January 1969 after a heated exchange with the then manager Stan Anderson.
That year we got promoted, and then at the beginning of the following season Andy Nelson, who had taken over from Theo, phoned me and told me he had sold me to Watford.
[...] I was put under some pressure to go to Watford, and various people I spoke to advised me to ask for outrageous wages in the hope that they would knock me back.
I thought it was wrong that I was phoned up at home to be told that I was meeting their manager the next morning – a 'thanks and goodbye' type of attitude.
Horsfield, who lived in Kent, had signed to Watford on the understanding that he wouldn't be asked to move home so as to not interrupt his children's schooling.
I had taken a coaching badge at Middlesbrough when I was 17, along with Bobby Braithwaite, Eddie Connachan and Taffy Orrit, but I wasn't ready to do it full time.
"[1]After being contacted by Watford chairman Elton John's uncle; former Nottingham Forest cup-finalist Roy Dwight, he transferred to Dartford to play in the Southern League.