Threlfall made his debut for Blackpool in the opening League game of the 1900–01 season, a goalless draw at New Brighton Tower. He went on to appear in 29 of their remaining 33 League games that season.[1] He also became possibly the first Blackpool player to be sent off in a League game. During the fixture against Burslem Port Vale on 15 December, Threlfall was given his marching orders for "rough play"[2] when "a regrettable incident occurred about three minutes off time".[2] Threlfall later complained that he had not been cautioned "before the referee took this extreme course".[2]
In 1901–02, Threlfall missed only two of Blackpool's 34 League games. He also scored his first goal for the club, in a 1–1 draw at home to Glossop North End on 1 February.[3]
He made 26 League appearances in 1902–03, including one game at right-back while Bob Birkett was unavailable. He scored two goals — the first in a 2–1 defeat at Woolwich Arsenal on 8 November; the second in a 2–0 victory over Manchester United at Bloomfield Road on 13 February.[4]
Threlfall chalked up thirty League appearances in 1903–04, scoring one goal — in a 2–1 victory at home to Gainsborough Trinity on 19 March.[5]
He scored the fifth goal of his career the following season, 1904–05, in a 6–0 home victory over Barnsley on 7 January, en route to making 32 League appearances. He also made his FA Cup debut for the club on 14 January, in a 2–1 defeat at Bristol City.[6]
Threlfall missed only one game of Blackpool's 1905–06 League campaign, scoring one goal in the process — in a 3–0 home win against Clapton Orient on 27 April, in the final League game of the season. He also scored once in the FA Cup to take Blackpool to a second first-round replay against Crystal Palace.[7]
In 1906–07, Threlfall made 36 League appearances. He also scored three goals, all at Bloomfield Road — in a 4–1 victory over Glossop North End on 20 October, in a 2–1 defeat to Nottingham Fores on 24 November, and in a 4–3 victory over Grimsby Town on 20 April, in the penultimate League game of the season.[8]
Starts for Threlfall were sporadic in 1907–08. He made twenty League appearances in total, but he sat out fifteen consecutive games between November and February. He scored one goal, in a 3–0 home win over Grimsby Town on 1 April.[9]
Threlfall made 28 League appearances in 1908–09. He scored one goal, in Blackpool's 2–0 FA Cup first-round victory over Hastings & St Leonards United on 16 January.[10]
In 1909–10, his tenth season with Blackpool, Threlfall made 31 League appearances.[11]
1910–11 proved to be Threlfall's final season with Blackpool. He made eighteen League appearances, the final one occurring on 25 February, in a 2–1 defeat at Clapton Orient.[12]