In his playing days, Ternent donned the colours of Burnley and Carlisle United. Upon retiring, he became a coach at Sunderland, firstly, then Blackpool, assisting Bob Stokoe. Ternent himself became manager of the Tangerines in 1979, his first such role, and Blackpool's sixth manager in a decade.
Immediately upon his appointment at Bloomfield Road, Ternent began to reshape the team, spending large sums on new players. Jack Ashurst was purchased from Sunderland for a then-club-record £132,400. Fellow newcomers included Dave Bamber, Colin Morris, Peter Noble and Tom McAlister. Despite the fresh faces, the Seasiders' fortunes didn't improve, and by early 1980 they were in the bottom half of the Third Division. Ternent was sacked on 1 February 1980, at the time becoming the club's shortest-serving manager in their history.
Ternent was not out of the game for long. He moved from the Premier League to Division Three in a matter of weeks, becoming Mike Walsh's right-hand man at Bury. When Walsh quit in October 1995, Ternent moved up to the manager's seat, and seven months later he took them to Division Two as third-placed team in Division Three. A year later they won the Division Two championship and survived the first season in Division One; however, Ternent left Gigg Lane at the end of the 1997–98 season to take charge of Burnley in Division Two.
In Ternent's second season as Clarets manager, 1999–2000 and after a clear out, the club finished second in Division Two and won automatic promotion. He remained in charge for another four seasons, but the club's board did not renew his contract after the 2003–04 campaign and he left the club after six years, having failed to get them into the Premier League.
Ternent made a brief comeback in 2004–05 with Gillingham, taking Ronnie Jepson from Burnley to be his assistant; however, his short-term contract as manager was not renewed, as despite a heroic attempt at survival they were relegated to League One. Ternent quit Gillingham in May 2005.
In November 2007, he was appointed to the backroom staff of new Derby County manager Paul Jewell. He left this role on 24 April 2008 to take up the reins at Huddersfield Town in place of Andy Ritchie, who had been relieved of his duties at the beginning of the month. Ex-Town player Ronnie Jepson, appointed on the same day, became his assistant. He officially started his new job on 28 April, leaving Gerry Murphy in charge of Town's last home game against Walsall on 26 April and their last away game at Luton Town on 3 May.