[6] Grego-Cox was included in a matchday squad for the first time on 4 January 2015, remaining an unused substitute as they lost 0–3 to League One side Sheffield United at Loftus Road in the third round of the FA Cup.

[7] He made his professional debut on 7 March, replacing Karl Henry for the last two minutes of a 1–2 home defeat against Tottenham Hotspur in the Premier League.

[10] He made his debut on 3 September by starting at home to Barnet but the match was abandoned at half time due to a waterlogged pitch.

On 16 July 2018, Grego-Cox made the move to League Two side Crawley Town on a two-year deal, following a successful trial period.

On 13 November 2014, Grego-Cox made his debut for the Under-19s as they hosted their group in the qualifying round for the 2015 European Championship, starting in a 1–0 win over Malta at the Regional Sports Centre in Waterford.

[25] Two days later, he opened the scoring in the 3rd minute of a 4–1 victory against Gibraltar at Ferrycarrig Park in Wexford,[26] and on 18 November in Waterford he netted the equaliser for a 1–1 draw with Switzerland.

On 4 September 2015, Grego-Cox made his under-21s debut, coming on as a second-half substitute for Sean Kavanagh in their 4–1 win over Qatar at Gigg Lane in Bury; he scored their third goal.

They noted how his "pace and aggressive style of play" had proven successful with the international under-17 team, but acknowledged concerns over his temperament, despite concluding that "a competitive edge didn't do Roy Keane much harm".

[3] Queens Park Rangers under-18 manager Steve Gallen compared Grego-Cox to first-team player Charlie Austin in August 2014, saying "They are tenacious, they cause problems for defenders and they can score goals.