He remained with Newcastle for five years, making more than 150 appearances in all competitions before finishing his professional career with Third Division South side Southend United.

[8][9] His spell with the team was disrupted by a broken leg during his second season,[9] however he was still chosen to represent an Irish League XI on several occasions and captained the side against their English counterparts.

[11] Nelson moved to Wales in 1921 when signed by Cardiff City, coinciding with the team being promoted to the First Division of the Football League.

[12] He made his debut for the club two months into the 1921–22 season, replacing captain Charlie Brittain in a 2–0 victory over West Bromwich Albion on 29 October.

[15] In the 1925–26 season he remained prominent in the first team and scored his first goals for the club, converting penalties during victories over Burnley and Nottingham Forest in the second half of the campaign.

The season culminated with Nelson and Cardiff both reaching their first FA Cup final, losing 1–0 to Sheffield United at Wembley Stadium.

[16] In the opening game of the 1925–26 season, a 3–2 defeat to Manchester City, Nelson became the first Cardiff player to be sent off in a Football League match.

He was subsequently dismissed after the referee witnessed him kick opposition forward Tommy Johnson, with Manchester City converting the resulting penalty to win the game.

[19][20] Nelson missed the opening four matches of the 1926–27 season before returning to the side and remaining ever present for the remainder of the campaign, appearing in 50 consecutive games as Cardiff finished 14th in the First Division.

At the end of the season, he helped the side reach the 1927 FA Cup Final against Arsenal,[21] shortly before which the Daily Herald described him as "one of the best backs in the country".

After featuring in the opening nine games the following campaign, he succumbed to injury and appeared only two more times as Cardiff suffered relegation to the Second Division after finishing last.

He won a fourth Welsh Cup title during his final season, scoring his last goal for the club in a 4–0 victory over Swansea City, their local rivals, in the sixth round.

[27] Nelson was still held in high esteem in South Wales from his spell with Cardiff and a benefit night was hosted in his honour in Ystrad Mynach shortly after the cup final.

[35] Nelson made his international debut for Scotland on 14 February 1925 in a 3–1 victory over Wales, a match in which he was the only Scottish player from outside his nation's league system to be selected.

His son Tony became an amateur international footballer for Wales then later turned professional with Newport County and AFC Bournemouth.