Newcastle United Cass joined the
Academy at
Newcastle United from North Shields Juniors at the age of 12. He was next named on the bench by new manager
Steve Bruce in an
FA Cup tie with
Rochdale in January 2020. He made his first-team debut two days later as "Pools" recorded a 2–1 win over
Aldershot Town. He played primarily on the right-hand side of a three-man defence at Hartlepool. On 9 March, he was
sent off 24 minutes into a 1–1 draw at
Altrincham. He made a total of 37 appearances throughout the campaign, helping Hartlepool to qualify for the play-off quarter-finals in fourth-place. Hartlepool manager
Dave Challinor tried to bring back Cass to
Victoria Park on loan for the following season but was unable to do so due to competition from other clubs.
Port Vale On 12 July 2021, Newcastle United loan manager
Shola Ameobi sanctioned a season-long loan move to
League Two side
Port Vale. He was signed by manager
Darrell Clarke to compete with
James Gibbons at right-back and to provide cover at centre-back. He made his debut for the "Valiants" on 10 August, coming on as a 65th-minute
substitute for
Dan Jones in a 2–1 defeat to Newcastle's
Tyne–Wear derby rivals
Sunderland in an
EFL Cup first round fixture at
Vale Park. He scored his first goal in professional football in a 5–1 victory over
Accrington Stanley in an FA Cup first round match on 6 November. He impressed in his 27 appearances for the "Valiants", playing on the right of the back three or at wing-back, and received interest from other lower league clubs in the January
transfer window before his season was cut short after he tore his ankle ligaments in training on 27 January. During his recovery time he provided
scouting reports for Port Vale acting manager
Andy Crosby in the
North East. Port Vale went on to win promotion out of the
play-offs in May, and Cass said that "I hate watching this on TV, I would rather be a part of it". On 11 June 2022, Port Vale announced that Cass would join the club on a contract of undisclosed length from 1 July;
David Flitcroft, the club's
director of football, stated that "I spoke to Darrell [Clarke],
Andy [Crosby] and
Dean [Whitehead] and the way they all spoke about him you could see they admired him... it was a collective decision". He started four of the opening five league games of the
2022–23 season, before dropping out of the team to accommodate
Will Forrester. He missed February and March after being sidelined with a
calf strain. On 4 November 2023, he put in a
man of the match performance in a 0–0 draw at home to
Burton Albion after centre-back partner
Nathan Smith was
sent off, and after the game commented that he felt his physicality had greatly improved over the previous 12 months as he adapted to
League One football. However, seven days later he made a mistake to give away the equalising goal in a 1–1 draw at
Lincoln City, only to come off the bench to score his second career goal in the
replay at Burton Albion four days on from the Lincoln game. On 1 February 2024, Cass signed for
League Two club
Stockport County until the end of the
2023–24 season, reuniting with his former boss at Hartlepool, Dave Challinor. He scored his first ever league goal on his debut for the club nine days later, in a 3–1 win at
Grimsby Town. On 9 March, he won nine aerial duels and also made four clearances and two interceptions as Stockport beat
Newport County 1–0; his performance earned himself a place in the EFL Team of the Week. Stockport were promoted as champions and he returned to Port Vale, who had been relegated out of League One. He returned to Port Vale, where he was released.
Grimsby Town On 9 July 2024, Cass signed a two-year contract with League Two club
Grimsby Town. Head coach
David Artell said he "had a longstanding interest in Lewis" and had tried to sign him in the January transfer window. He made his competitive debut for the club in a 1–0 away defeat to
Fleetwood Town on the opening day of the season. Cass scored his first goal for the club as a substitute in a 3–2 victory over
Carlisle United on 28 September. He played 31 games in the
2024–25 campaign, including 18 league starts. Having secured Cass to a two-year deal, Head of Recruitment
Lloyd Kerry said that he and manager
Simon Weaver had tried to sign him previously and had been monitoring him ever since. Cass struggled with injuries thoughout the
2025–26 season, which culminated in relegation for the club. ==Style of play==