Maddren made his debut on 12 April 1969 against Bury scoring a goal and suffering a broken nose. Despite starting his Boro career as a striker he was known as a classy central defender. In the 1970–71 season Maddren established himself in the Boro defence. In 1971 Middlesbrough manager Stan Anderson bought Mansfield Town central defender Stuart Boam to form a formidable defensive partnership with Maddren. In 1973, after repeated failures to gain promotion to the old First Division, Anderson left Boro and was replaced by World Cup winner Jack Charlton.
In the 1973–74 season Middlesbrough ran away with the old Second Division finishing 15 points clear from second place Luton Town (Under the old points system of 2 points for a win.) They were promoted to the First Division on 23 March 1974, finishing the season conceding only 30 goals, keeping 25 clean sheets in a 42-game programme. In the old First Division Boro were a strong, competent team just short of being top-class. The lack of a proven goalscorer was a factor in Boro failing to qualify for Europe in 1974–1975 by one place. Maddren claimed in later years that if Boro had bid for Malcolm Macdonald they would have challenged for the Championship. His manager concurred. Jack Charlton said his biggest regret in football was not winning the Championship in 1974–75.
Maddren played for England under-23's five times, but despite a call-up to the full England side he never played. This was a source of bitterness for Maddren and many commentators agreed with him. Maddren claimed if he played for a more glamorous team than Middlesbrough he would have played for England. He compared his treatment with that of Brian Greenhoff (Manchester United, Leeds United) who won 18 caps despite being less talented. When BBC Radio Five Live announced Maddren's death on its sports news in 2000, a guest on the station, football journalist Hugh McIlvanney claimed he was the best player never to play for England. Maddren's final appearance for Boro was on 3 September 1977 against West Bromwich Albion. He was only 26. Maddren had a persistent knee injury and he often played games when in pain. An operation was unsuccessful and he retired in 1979.
When Malcolm Allison became Middlesbrough manager in 1982 Maddren returned to Ayresome Park as physiotherapist. By then Boro were struggling in the old Second Division. Allison was sacked on 24 March 1984, and was replaced by Jack Charlton as caretaker manager with Maddren as assistant. The new management team staved off relegation and Maddren was promoted to post of manager at the season's end.
This period brought financial difficulties which eventually led to liquidation in 1986 and the near loss of professional football on Teesside. At the end of 1985–86 season Boro were relegated to the old Third Division. By then Maddren was no longer manager. In his first full season 1984–85 Boro struggled and avoided relegation on the last day of the season, 11 May 1985 in Shrewsbury.