As a player, he was a defender for Walsall, Watford, Bradford City, Crystal Palace, Huddersfield Town, Oldham Athletic and Scarborough.
As manager of Farsley Celtic between 2003 and 2007, he won promotion three times in four seasons to take the club from the Northern Premier League to the Conference.
[4] He made 40 first-team appearances in his first season at Fellows Park before he was sold to First Division runners-up Watford for a £130,000 fee in September 1983.
[5] At the end of his first season, when still only 18 years old, he played in the 1984 FA Cup final against Everton at Wembley Stadium, collecting a runners-up medal.
[5] He helped Watford reach the semifinals of the FA Cup in the 1986–87 campaign and played at left-back across from Chris Waddle in the defeat to Tottenham Hotspur.
He spent two more seasons in West Yorkshire before crossing the Pennines and signing for local rivals Oldham Athletic, who had just been relegated to the Second Division.
Sinnott played 31 games over two seasons at Boundary Park, but Oldham struggled in a division from which they had been expected to win promotion.
Upon its conclusion, he moved to Scarborough, who had just been relegated to the Conference and played 22 games in one season for a side that finished fourth in the country's highest non-League division.
The other clubs in the promotion mix appealed and the FA reversed their decision to expunge the results, leaving Farsley in third place and having to compete in the play-offs.
This time, Farsley finished fourth in the regulation season before defeating Marine 1–0 and then North Ferriby United 2–1 in the final.
Sinnott also added the West Riding County Cup to Farsley's trophy cabinet for the tenth time in the club's history, as they made it a double-winning campaign in 2005–06.
Despite finishing fifth in the Conference North, Lee lead the club to victory in the playoffs, ensuring Farsley Celtic's first-ever appearance in the top flight of Non-League.
[10] Prominent backroom staff member Dean Glover had hoped to gain the management spot himself, and so pondered resigning from the club,[11] but eventually chose to remain as Sinnott's assistant.
To make a success of the 2008–09 season, Sinnott brought in former Hereford United defender John McCombe,[19] and former Middlesbrough midfielder Steve Thompson.
[25] Lee Collins also joined on an extended loan from Wolverhampton Wanderers,[26] and would sign a permanent contract with the club in January.
Sinnott was linked with a vacant position at newly-promoted Northern Premier League side Bradford Park Avenue at the end of 2008.
The club were unwilling to pursue the interest because of Sinnott's unresolved contract with Port Vale,[33] and instead appointed his former Farsley assistant and successor John Deacey.
"[38] He left by mutual consent in October 2009, after a summer spending spree failed to produce results on the pitch for the 2009–10 season.
[44] On 13 February 2018, Sinnott was appointed as manager of Gainsborough Trinity, who were then just inside the National League North relegation zone.
He remained at The Northolme for the start of the 2018–19 campaign but was sacked exactly one year into his appointment following a 5–0 defeat by league leaders Farsley Celtic which left Trinity four points outside the play-offs.