He also appeared regularly for Watford's reserve team, and was rewarded for his performances with a one-year professional contract at the end of the season.

Despite being heavily linked with a permanent move to Scottish club Peterhead, he also signed a one-year contract extension with Watford on the same day.

[9] Assombalonga made an eventful debut in a 3–3 draw against Lincoln City; he scored his first goal for the club to put Braintree 3–2 up before being sent off for a second bookable offence.

[10] He scored four more goals in four games for Braintree after serving his one-match suspension, and was subsequently recalled by Watford to join their senior squad.

[18] On 31 July 2013, League One side Peterborough United broke their previous transfer record to sign Assombalonga from Watford on a four-year deal.

[22][23][24] This made Assombalonga Forest's record signing, surpassing the £4.5 million fee paid to Celtic for striker Pierre van Hooijdonk in 1997.

[25] Assombalonga scored his first goals for Forest twelve days after signing, netting a brace against Bolton Wanderers at the Macron Stadium.

[29] He then went on to score in the reverse fixture against Derby, equalising at Pride Park as Forest secured a 2–1 win to relieve the pressure on their beleaguered manager Stuart Pearce.

On 11 February 2015, Assombalonga sustained a serious knee injury during a 3–0 victory over Wigan Athletic after taking a shot at goal that prematurely ended his season.

[32] On the opening day of the 2016–17 season, Assombalonga started the first competitive match under new manager Philippe Montanier and scored a brace at home against newly promoted Burton Albion.

[42] Middlesbrough did not offer him a renewal of his contract, due to expire at the end of the 2020–21 season, and manager Neil Warnock confirmed on 21 April 2021 that he had been allowed to leave the club early.

[51] That December, Assombalonga rejected the chance to represent DR Congo at the 2017 Africa Cup of Nations in Gabon, stating that he wanted to focus on playing games for Nottingham Forest after an injury-hit season.