He then scored twenty goals at League One level throughout the next two seasons, securing a £750,000 transfer move to Ipswich Town in July 2018.

Harrison began his career at Bristol Rovers and made his senior debut for the Pirates in League One on 16 April 2011, in a 1–0 defeat to Southampton at St Mary's Stadium, when he came on as a 75th-minute substitute for Jo Kuffour.

[3] This was the first time he had been in a first-team matchday squad, having been included after impressing caretaker manager Stuart Campbell during a reserve team game four days earlier.

Harrison was sent off in the play-off semi-final game with Forest Green Rovers,[14] but returned for the final at Wembley Stadium where he scored the crucial equalising goal against Grimsby Town.

[16][17] On 17 January 2016, he agreed to join fellow League Two side Hartlepool United on a month-long loan to attain more game time.

[19] As Ellis returned to Rovers from his loan spell, he scored a crucial late equaliser against league leaders Northampton Town.

[20] This proved to be important for the club's automatic promotion race, as Rovers went up on goal difference with a victory over Dagenham & Redbridge on the final day.

[23] He won the January 2018 goal of the month award for League One with his solo effort against Doncaster Rovers late on in a 3–1 victory.

[28] However, Harrison was dropped after five games and played out of position on the left side of midfield before he suffered an ankle ligament injury in training in September, which ruled him out of action for two months.

[31] That would be his only goal for the Tractor Boys during his nine starts and eight substitute appearances throughout the 2018–19 relegation campaign as he was struck down with a back injury in February.

[29] Harrison scored his first Portsmouth goal in his second game, a 3–0 victory over Birmingham City in an EFL Cup first round fixture at Fratton Park on 6 August.

[36] He scored ten goals in 39 appearances during the 2019–20 season, which was stopped early due to the COVID-19 pandemic in England, with Portsmouth finishing in the play-offs on points per game.

[39] In June 2021, Harrison looked set to join Oxford United, however, this move fell through following Portsmouth's failure to bring in replacement Jayden Stockley.

[43] He scored a total of four goals in fourteen appearances in the first half of the 2021–22 season, though he started just one league fixture, and opted to leave the club after manager Danny Cowley told him he would not be offered a new contract in the summer.

[52] He developed an effective strike partnership with James Wilson and was named as Vale's Player of the Month for both August and September.

[67] He scored his first international goal while gaining his second cap in the 4–0 win over San Marino four days later in a 2015 UEFA European Under-21 Championship qualification game at Nantporth.