Westley was appointed manager of Conference National club Rushden & Diamonds in December 2006, with the club placed 22nd in the league table, two points adrift of safety. Westley's first game as manager of Rushden was a 1–0 victory away at Cambridge United on 26 December 2006. He signed several players who he had previously managed at Stevenage; signing Jamie Cook, David Perpetuini, and Dino Maamria. Westley also brought in Michael Bostwick and Chris Beardsley, younger players who he had previously written about in his weekly column in The Non-League Paper. The team won five consecutive games in the league, including victories away at the top two teams in the league at the time, Oxford United and Dagenham & Redbridge. Westley was sacked after a 2–2 draw at Aldershot Town in February 2007. He had taken the club from 22nd in the league to 12th in the space of two-and-a-half months, and the club were on a seven-match unbeaten run at the time. Westley managed Rushden for 13 games; winning seven, drawing three and losing three. No reasons were given as to why Westley left the club and he was replaced two days later by Garry Hill.

After leaving Rushden, Westley was appointed caretaker manager of Conference North club Kettering Town for the rest of the 2006–07 season on 20 April 2007. His first game as manager was a 0–0 draw away to Stalybridge Celtic. Kettering finished the season in second place and they were beaten on penalties by Farsley Celtic in the play-off semi-final after a 1–1 aggregate scoreline over two legs. He left Kettering when his short-term contract expired in May 2007, managing the club for four games.

After almost a year without work in football, Westley returned as manager of Stevenage on 2 May 2008, two years after he had left. On his return, Westley stated he had come to "finish the job he started", referring to trying to guide the club into the Football League for the first time in its history. He said that "the moment was right" for him to return to football management and that he "had some very good times at Stevenage before". His appointment was met with scepticism by some Stevenage supporters, but Westley promised to deliver a winning team. Westley started by overhauling the squad in the summer transfer window and brought in 12 players. Among those brought in were Gary Mills and David Bridges, who Westley had managed during his time at Kettering. He also signed Michael Bostwick and Mark Albrighton, who he had also signed at Rushden & Diamonds.

In January 2012, Preston North End asked for permission to speak to Westley with the view to employing him as their new manager. Though the Hertfordshire club described the approach as "unwelcome", permission was granted. After personal terms and a compensation package were agreed, Westley was appointed as Preston's new manager on 13 January 2012. His first game in charge of the club was a 2–0 home defeat against Leyton Orient on 21 January 2012. He earned his first win as Preston manager in his fourth match in charge, securing a 1–0 home victory over Hartlepool United on 14 February 2012. Following Preston's 2–0 televised defeat to Sheffield Wednesday on 31 March 2012, Westley stated that a Sheffield Wednesday player had informed him that four Preston players had leaked the Preston team and tactics to opposition players ahead of the match. Westley stated "It doesn't surprise me. When you have got people in your own camp working against you it is tough". Preston won two games out of the next 17 during the remainder of the season, and finished in 15th place. At the end of the season, Westley criticised the club's "mediocre" mentality, stating the squad have "not got a clue what it takes to get success".

After a month out of work, Westley rejoined Stevenage on 30 March 2013, his third spell as manager at the club. On the appointment, Stevenage chairman Phil Wallace stated – "Graham was keen to come back to Stevenage and, although I met some strong candidates, without doubt I think he's the best man for the job right now. The club enjoyed a great deal of success before he left last year and we moved forwards significantly in his time here on several levels". On his return, Westley stated "I'm really happy to be back, the training ground is fantastic, the club just keeps moving forward. All the facilities keep improving and I hope that I can come back and make another positive difference". In his first game back in charge of the club, Stevenage secured a 1–0 victory over Hartlepool United on 1 April 2013. Stevenage ended the 2012–13 season in 18th place, and Westley released four players and transfer-listed three others.