Born in Halifax, West Yorkshire,[2] Senior started out at Elland Juniors before joining the youth system at Huddersfield Town aged 10.[5] He began a two-year scholarship at the start of 2013–14,[6] before signing his first professional contract with the club for 2015–16.[7] Senior was involved in the first-team pre-season programme,[8] but predominantly played for the Development Squad that won the Northern Division by eight points, for whom he scored two goals from 36 appearances.[9] At the end of the season, Huddersfield took up the option to extend Senior's contract by a further year.[10]
Luton Town
Senior signed for League Two club Luton Town on a two-year contract on 31 August 2016.[11] After signing for Luton, manager Nathan Jones said "He ticks all the boxes for us. He's a young, hungry and athletic player who wants to learn."[11] He suffered an injury setback in training,[12] though he made his Luton debut on 4 October, starting in their 2–0 victory at home to West Bromwich Albion U21 in the EFL Trophy.[5][13] Senior made his English Football League debut as a 56th-minute substitute for Jordan Cook in a 2–1 win away to Crewe Alexandra on 14 January 2017.[14] Thereafter, he made 10 consecutive starts,[15] which ended when he was rested for Luton's league match against Yeovil Town on 4 March.[16] Senior started in Luton's 1–1 draw at home to Exeter City two weeks later,[17] but missed their subsequent four matches after he was sidelined with a foot injury.[18] In April 2017, he was named in the EFL Trophy Team of the Tournament.[19] Despite being available for selection, Senior made no further appearances and finished 2016–17 with 16 appearances.[15]
On 14 November 2017, Senior signed a one-year contract extension with Luton, with the option of a further year.[20] He joined National League club Harrogate Town on 31 January 2019 on loan until the end of 2018–19.[21] Senior was released by Luton when his contract expired at the end of the 2018–19 season.[22]