His seven goals in four matches in the FA Cup saw him finish as top scorer in the competition. This included four goals in a 5–0 win away to Staines Town in the second round, which saw him win the FA Cup Player of the Round award. The following season, Mackail-Smith scored 23 goals in 46 appearances to help Peterborough win a second successive promotion to the Championship. Steve Claridge published a scouting report on Mackail-Smith on 23 September 2008. He described him as "a rampaging Nordic Viking with his long blonde hair" and was impressed by the way he would pick up the ball and run at defenders, attacking at every opportunity in what Claridge described as a positive way. Mackail-Smith scored 35 goals for the club in 2010–11, making him the most prolific scorer in England for the season. His 27 goals in League One made him top scorer and earned him the League One Player of the Year award, helping Peterborough to reach the League One play-offs. He was also named in the PFA League One Team of the Year. Mackail-Smith scored in each play-off match, including the second goal in a 3–0 win over Huddersfield Town in the 2011 Football League One play-off Final at Old Trafford to clinch an instant return to the Championship for Peterborough. Following the match, Mackail-Smith expressed his desire to leave the club, stating "I want to play at the highest level I can attain and I want to play in the Premier League." He was the subject of bids from Premier League clubs including Queens Park Rangers. Mackail-Smith made 212 appearances for Peterborough United, scoring 99 goals. On 4 July 2011, Mackail-Smith joined Brighton & Hove Albion for an initial transfer fee of £2.5 million, possibly rising to £3.25 million, signing a four-year contract. He scored his first goal for Brighton in a 1–0 win away to Portsmouth on 13 August. Mackail-Smith then scored four goals in his next seven matches, including a brace in a 3–3 draw at home to Leeds United. However, Mackail-Smith then hit a goal drought and struggled to score, netting just one goal between October and February in a 1–0 win away to Derby County on 29 November. He ended his goal drought by coming off the bench to score the opening goal in a 2–1 win away to Leeds United, against whom he scored a brace earlier in the season.